Prevent warnings on dictionaries w/o string keys
Prevent warnings on dictionaries w/o string keys

Because the category makes the protocol apply to all `NSMutableDictionaries`, it can't assume keys are `NSString`. Because these types are covariant, the change is not breaking.
wcevans/grpc,greasypizza/grpc,stanley-cheung/grpc,daniel-j-born/grpc,firebase/grpc,soltanmm/grpc,kriswuollett/grpc,adelez/grpc,arkmaxim/grpc,mehrdada/grpc,kriswuollett/grpc,jcanizales/grpc,thinkerou/grpc,kumaralokgithub/grpc,soltanmm-google/grpc,wcevans/grpc,soltanmm-google/grpc,msiedlarek/grpc,andrewpollock/grpc,jtattermusch/grpc,kriswuollett/grpc,grani/grpc,dklempner/grpc,simonkuang/grpc,donnadionne/grpc,kumaralokgithub/grpc,podsvirov/grpc,podsvirov/grpc,chrisdunelm/grpc,ctiller/grpc,msiedlarek/grpc,thinkerou/grpc,VcamX/grpc,goldenbull/grpc,daniel-j-born/grpc,kumaralokgithub/grpc,ncteisen/grpc,matt-kwong/grpc,daniel-j-born/grpc,dklempner/grpc,vsco/grpc,mehrdada/grpc,yongni/grpc,PeterFaiman/ruby-grpc-minimal,baylabs/grpc,tengyifei/grpc,simonkuang/grpc,dgquintas/grpc,jtattermusch/grpc,kpayson64/grpc,7anner/grpc,pszemus/grpc,Vizerai/grpc,jboeuf/grpc,LuminateWireless/grpc,dgquintas/grpc,MakMukhi/grpc,andrewpollock/grpc,hstefan/grpc,fuchsia-mirror/third_party-grpc,arkmaxim/grpc,bogdandrutu/grpc,vjpai/grpc,goldenbull/grpc,jboeuf/grpc,chrisdunelm/grpc,Vizerai/grpc,y-zeng/grpc,arkmaxim/grpc,y-zeng/grpc,jtattermusch/grpc,jtattermusch/grpc,arkmaxim/grpc,andrewpollock/grpc,ctiller/grpc,bogdandrutu/grpc,sreecha/grpc,deepaklukose/grpc,ananthonline/grpc,ncteisen/grpc,vjpai/grpc,chrisdunelm/grpc,vsco/grpc,mehrdada/grpc,thinkerou/grpc,msiedlarek/grpc,rjshade/grpc,baylabs/grpc,LuminateWireless/grpc,dgquintas/grpc,Vizerai/grpc,carl-mastrangelo/grpc,jcanizales/grpc,hstefan/grpc,adelez/grpc,PeterFaiman/ruby-grpc-minimal,stanley-cheung/grpc,deepaklukose/grpc,apolcyn/grpc,perumaalgoog/grpc,vjpai/grpc,hstefan/grpc,ejona86/grpc,infinit/grpc,MakMukhi/grpc,stanley-cheung/grpc,firebase/grpc,yang-g/grpc,kpayson64/grpc,murgatroid99/grpc,a11r/grpc,ppietrasa/grpc,baylabs/grpc,philcleveland/grpc,leifurhauks/grpc,jboeuf/grpc,apolcyn/grpc,ctiller/grpc,stanley-cheung/grpc,VcamX/grpc,ipylypiv/grpc,muxi/grpc,tamihiro/grpc,murgatroid99/grpc,carl-mastrangelo/grpc,pmarks-net/grpc,goldenbull/grpc,greasypizza/grpc,jtattermusch/grpc,bogdandrutu/grpc,hstefan/grpc,kpayson64/grpc,7anner/grpc,makdharma/grpc,rjshade/grpc,thinkerou/grpc,kskalski/grpc,wcevans/grpc,pszemus/grpc,rjshade/grpc,tamihiro/grpc,bogdandrutu/grpc,muxi/grpc,fuchsia-mirror/third_party-grpc,miselin/grpc,y-zeng/grpc,pmarks-net/grpc,kriswuollett/grpc,soltanmm/grpc,simonkuang/grpc,thunderboltsid/grpc,7anner/grpc,grpc/grpc,dgquintas/grpc,muxi/grpc,pszemus/grpc,ctiller/grpc,yang-g/grpc,ejona86/grpc,adelez/grpc,royalharsh/grpc,grani/grpc,tengyifei/grpc,wcevans/grpc,thunderboltsid/grpc,fuchsia-mirror/third_party-grpc,wcevans/grpc,kriswuollett/grpc,adelez/grpc,grani/grpc,ncteisen/grpc,geffzhang/grpc,kriswuollett/grpc,baylabs/grpc,VcamX/grpc,vsco/grpc,simonkuang/grpc,nicolasnoble/grpc,perumaalgoog/grpc,PeterFaiman/ruby-grpc-minimal,Vizerai/grpc,soltanmm/grpc,pmarks-net/grpc,jcanizales/grpc,nicolasnoble/grpc,Vizerai/grpc,bogdandrutu/grpc,ncteisen/grpc,dgquintas/grpc,MakMukhi/grpc,soltanmm-google/grpc,MakMukhi/grpc,ppietrasa/grpc,miselin/grpc,firebase/grpc,malexzx/grpc,Crevil/grpc,tamihiro/grpc,makdharma/grpc,sreecha/grpc,andrewpollock/grpc,infinit/grpc,matt-kwong/grpc,kskalski/grpc,wcevans/grpc,ppietrasa/grpc,muxi/grpc,firebase/grpc,baylabs/grpc,philcleveland/grpc,miselin/grpc,wcevans/grpc,pszemus/grpc,zhimingxie/grpc,vjpai/grpc,mehrdada/grpc,ipylypiv/grpc,VcamX/grpc,quizlet/grpc,pmarks-net/grpc,grpc/grpc,hstefan/grpc,podsvirov/grpc,a11r/grpc,kpayson64/grpc,apolcyn/grpc,andrewpollock/grpc,infinit/grpc,dklempner/grpc,ppietrasa/grpc,carl-mastrangelo/grpc,7anner/grpc,pszemus/grpc,apolcyn/grpc,yang-g/grpc,tengyifei/grpc,infinit/grpc,greasypizza/grpc,vjpai/grpc,a-veitch/grpc,thinkerou/grpc,ipylypiv/grpc,soltanmm-google/grpc,mehrdada/grpc,nicolasnoble/grpc,goldenbull/grpc,grani/grpc,matt-kwong/grpc,carl-mastrangelo/grpc,mehrdada/grpc,philcleveland/grpc,a-veitch/grpc,quizlet/grpc,zhimingxie/grpc,hstefan/grpc,soltanmm/grpc,quizlet/grpc,thunderboltsid/grpc,kskalski/grpc,geffzhang/grpc,pszemus/grpc,Crevil/grpc,sreecha/grpc,ncteisen/grpc,miselin/grpc,donnadionne/grpc,vsco/grpc,jboeuf/grpc,kskalski/grpc,msmania/grpc,zhimingxie/grpc,msmania/grpc,andrewpollock/grpc,kriswuollett/grpc,vjpai/grpc,donnadionne/grpc,yongni/grpc,ipylypiv/grpc,royalharsh/grpc,carl-mastrangelo/grpc,7anner/grpc,greasypizza/grpc,ipylypiv/grpc,murgatroid99/grpc,mehrdada/grpc,royalharsh/grpc,quizlet/grpc,thinkerou/grpc,7anner/grpc,murgatroid99/grpc,Vizerai/grpc,leifurhauks/grpc,leifurhauks/grpc,perumaalgoog/grpc,pmarks-net/grpc,thinkerou/grpc,PeterFaiman/ruby-grpc-minimal,apolcyn/grpc,nicolasnoble/grpc,vsco/grpc,mehrdada/grpc,ejona86/grpc,MakMukhi/grpc,zhimingxie/grpc,murgatroid99/grpc,jcanizales/grpc,leifurhauks/grpc,tamihiro/grpc,Vizerai/grpc,thinkerou/grpc,kpayson64/grpc,jtattermusch/grpc,deepaklukose/grpc,dklempner/grpc,matt-kwong/grpc,yongni/grpc,kumaralokgithub/grpc,soltanmm-google/grpc,daniel-j-born/grpc,perumaalgoog/grpc,vjpai/grpc,ipylypiv/grpc,ncteisen/grpc,deepaklukose/grpc,arkmaxim/grpc,yugui/grpc,tamihiro/grpc,y-zeng/grpc,donnadionne/grpc,quizlet/grpc,yugui/grpc,royalharsh/grpc,carl-mastrangelo/grpc,perumaalgoog/grpc,rjshade/grpc,deepaklukose/grpc,kriswuollett/grpc,firebase/grpc,miselin/grpc,kumaralokgithub/grpc,infinit/grpc,makdharma/grpc,grpc/grpc,ctiller/grpc,dgquintas/grpc,goldenbull/grpc,kpayson64/grpc,dklempner/grpc,a-veitch/grpc,baylabs/grpc,yugui/grpc,vjpai/grpc,PeterFaiman/ruby-grpc-minimal,malexzx/grpc,yongni/grpc,Crevil/grpc,podsvirov/grpc,rjshade/grpc,leifurhauks/grpc,vjpai/grpc,grpc/grpc,matt-kwong/grpc,kpayson64/grpc,infinit/grpc,malexzx/grpc,firebase/grpc,grani/grpc,yugui/grpc,muxi/grpc,grani/grpc,kumaralokgithub/grpc,a11r/grpc,tengyifei/grpc,chrisdunelm/grpc,stanley-cheung/grpc,fuchsia-mirror/third_party-grpc,yang-g/grpc,infinit/grpc,dgquintas/grpc,grpc/grpc,malexzx/grpc,kpayson64/grpc,msmania/grpc,thunderboltsid/grpc,ctiller/grpc,bogdandrutu/grpc,carl-mastrangelo/grpc,yongni/grpc,firebase/grpc,VcamX/grpc,msiedlarek/grpc,msmania/grpc,jcanizales/grpc,y-zeng/grpc,daniel-j-born/grpc,miselin/grpc,a11r/grpc,pmarks-net/grpc,msmania/grpc,vsco/grpc,nicolasnoble/grpc,tamihiro/grpc,bogdandrutu/grpc,pszemus/grpc,ctiller/grpc,baylabs/grpc,soltanmm/grpc,fuchsia-mirror/third_party-grpc,muxi/grpc,ananthonline/grpc,ctiller/grpc,wcevans/grpc,kpayson64/grpc,ananthonline/grpc,soltanmm/grpc,PeterFaiman/ruby-grpc-minimal,perumaalgoog/grpc,yongni/grpc,arkmaxim/grpc,tengyifei/grpc,royalharsh/grpc,chrisdunelm/grpc,philcleveland/grpc,perumaalgoog/grpc,fuchsia-mirror/third_party-grpc,infinit/grpc,LuminateWireless/grpc,simonkuang/grpc,dklempner/grpc,jtattermusch/grpc,jtattermusch/grpc,carl-mastrangelo/grpc,PeterFaiman/ruby-grpc-minimal,muxi/grpc,makdharma/grpc,deepaklukose/grpc,soltanmm/grpc,jtattermusch/grpc,zhimingxie/grpc,grpc/grpc,Vizerai/grpc,miselin/grpc,ctiller/grpc,Crevil/grpc,ejona86/grpc,baylabs/grpc,podsvirov/grpc,sreecha/grpc,thunderboltsid/grpc,kumaralokgithub/grpc,ppietrasa/grpc,msiedlarek/grpc,ananthonline/grpc,stanley-cheung/grpc,greasypizza/grpc,ncteisen/grpc,grpc/grpc,jboeuf/grpc,donnadionne/grpc,carl-mastrangelo/grpc,hstefan/grpc,firebase/grpc,ejona86/grpc,malexzx/grpc,Vizerai/grpc,grpc/grpc,wcevans/grpc,7anner/grpc,mehrdada/grpc,hstefan/grpc,geffzhang/grpc,greasypizza/grpc,MakMukhi/grpc,jtattermusch/grpc,soltanmm-google/grpc,jboeuf/grpc,apolcyn/grpc,deepaklukose/grpc,VcamX/grpc,donnadionne/grpc,apolcyn/grpc,carl-mastrangelo/grpc,yang-g/grpc,stanley-cheung/grpc,nicolasnoble/grpc,stanley-cheung/grpc,muxi/grpc,adelez/grpc,ejona86/grpc,geffzhang/grpc,murgatroid99/grpc,dgquintas/grpc,stanley-cheung/grpc,sreecha/grpc,jtattermusch/grpc,adelez/grpc,donnadionne/grpc,makdharma/grpc,y-zeng/grpc,ipylypiv/grpc,sreecha/grpc,ejona86/grpc,msiedlarek/grpc,msiedlarek/grpc,thunderboltsid/grpc,philcleveland/grpc,Crevil/grpc,bogdandrutu/grpc,chrisdunelm/grpc,chrisdunelm/grpc,royalharsh/grpc,infinit/grpc,carl-mastrangelo/grpc,ctiller/grpc,vjpai/grpc,tamihiro/grpc,malexzx/grpc,greasypizza/grpc,PeterFaiman/ruby-grpc-minimal,soltanmm-google/grpc,LuminateWireless/grpc,quizlet/grpc,a11r/grpc,ananthonline/grpc,simonkuang/grpc,grani/grpc,jcanizales/grpc,andrewpollock/grpc,tengyifei/grpc,firebase/grpc,rjshade/grpc,LuminateWireless/grpc,LuminateWireless/grpc,ipylypiv/grpc,chrisdunelm/grpc,leifurhauks/grpc,VcamX/grpc,nicolasnoble/grpc,jboeuf/grpc,ncteisen/grpc,a11r/grpc,MakMukhi/grpc,donnadionne/grpc,thinkerou/grpc,ejona86/grpc,adelez/grpc,murgatroid99/grpc,kumaralokgithub/grpc,Crevil/grpc,pszemus/grpc,LuminateWireless/grpc,grpc/grpc,yugui/grpc,andrewpollock/grpc,podsvirov/grpc,grpc/grpc,stanley-cheung/grpc,a-veitch/grpc,miselin/grpc,murgatroid99/grpc,nicolasnoble/grpc,ppietrasa/grpc,yongni/grpc,vjpai/grpc,royalharsh/grpc,kskalski/grpc,fuchsia-mirror/third_party-grpc,tamihiro/grpc,Crevil/grpc,quizlet/grpc,kumaralokgithub/grpc,ctiller/grpc,carl-mastrangelo/grpc,chrisdunelm/grpc,kskalski/grpc,VcamX/grpc,msiedlarek/grpc,ananthonline/grpc,jboeuf/grpc,sreecha/grpc,tengyifei/grpc,yang-g/grpc,ejona86/grpc,murgatroid99/grpc,geffzhang/grpc,a11r/grpc,thinkerou/grpc,pszemus/grpc,ejona86/grpc,rjshade/grpc,pszemus/grpc,grani/grpc,fuchsia-mirror/third_party-grpc,quizlet/grpc,thinkerou/grpc,muxi/grpc,LuminateWireless/grpc,PeterFaiman/ruby-grpc-minimal,jcanizales/grpc,adelez/grpc,vjpai/grpc,yang-g/grpc,grpc/grpc,nicolasnoble/grpc,deepaklukose/grpc,donnadionne/grpc,bogdandrutu/grpc,sreecha/grpc,perumaalgoog/grpc,LuminateWireless/grpc,tengyifei/grpc,soltanmm/grpc,kskalski/grpc,philcleveland/grpc,ppietrasa/grpc,goldenbull/grpc,andrewpollock/grpc,perumaalgoog/grpc,baylabs/grpc,dgquintas/grpc,jboeuf/grpc,greasypizza/grpc,ananthonline/grpc,makdharma/grpc,malexzx/grpc,ctiller/grpc,nicolasnoble/grpc,msmania/grpc,makdharma/grpc,apolcyn/grpc,chrisdunelm/grpc,firebase/grpc,zhimingxie/grpc,jboeuf/grpc,soltanmm-google/grpc,malexzx/grpc,PeterFaiman/ruby-grpc-minimal,philcleveland/grpc,Crevil/grpc,geffzhang/grpc,nicolasnoble/grpc,apolcyn/grpc,dklempner/grpc,ncteisen/grpc,stanley-cheung/grpc,y-zeng/grpc,geffzhang/grpc,msmania/grpc,dgquintas/grpc,goldenbull/grpc,kpayson64/grpc,a-veitch/grpc,soltanmm-google/grpc,goldenbull/grpc,a-veitch/grpc,yang-g/grpc,yugui/grpc,miselin/grpc,arkmaxim/grpc,muxi/grpc,ananthonline/grpc,hstefan/grpc,jcanizales/grpc,malexzx/grpc,rjshade/grpc,msmania/grpc,philcleveland/grpc,Crevil/grpc,murgatroid99/grpc,arkmaxim/grpc,7anner/grpc,royalharsh/grpc,mehrdada/grpc,ppietrasa/grpc,pmarks-net/grpc,ncteisen/grpc,sreecha/grpc,adelez/grpc,thunderboltsid/grpc,daniel-j-born/grpc,leifurhauks/grpc,tamihiro/grpc,dklempner/grpc,sreecha/grpc,ncteisen/grpc,philcleveland/grpc,MakMukhi/grpc,ncteisen/grpc,geffzhang/grpc,kpayson64/grpc,ppietrasa/grpc,MakMukhi/grpc,pszemus/grpc,simonkuang/grpc,matt-kwong/grpc,thunderboltsid/grpc,kriswuollett/grpc,daniel-j-born/grpc,simonkuang/grpc,ananthonline/grpc,makdharma/grpc,msmania/grpc,y-zeng/grpc,grpc/grpc,Vizerai/grpc,thunderboltsid/grpc,fuchsia-mirror/third_party-grpc,deepaklukose/grpc,yongni/grpc,soltanmm/grpc,goldenbull/grpc,matt-kwong/grpc,firebase/grpc,simonkuang/grpc,kskalski/grpc,ipylypiv/grpc,podsvirov/grpc,vsco/grpc,arkmaxim/grpc,rjshade/grpc,yongni/grpc,zhimingxie/grpc,muxi/grpc,pszemus/grpc,dklempner/grpc,sreecha/grpc,matt-kwong/grpc,a-veitch/grpc,donnadionne/grpc,jtattermusch/grpc,Vizerai/grpc,a11r/grpc,donnadionne/grpc,makdharma/grpc,a-veitch/grpc,podsvirov/grpc,grani/grpc,kskalski/grpc,yang-g/grpc,greasypizza/grpc,donnadionne/grpc,mehrdada/grpc,pmarks-net/grpc,mehrdada/grpc,jboeuf/grpc,vsco/grpc,chrisdunelm/grpc,royalharsh/grpc,leifurhauks/grpc,7anner/grpc,thinkerou/grpc,nicolasnoble/grpc,firebase/grpc,ejona86/grpc,y-zeng/grpc,yugui/grpc,stanley-cheung/grpc,zhimingxie/grpc,jcanizales/grpc,leifurhauks/grpc,pmarks-net/grpc,VcamX/grpc,vsco/grpc,muxi/grpc,daniel-j-born/grpc,a11r/grpc,sreecha/grpc,daniel-j-born/grpc,fuchsia-mirror/third_party-grpc,msiedlarek/grpc,geffzhang/grpc,yugui/grpc,matt-kwong/grpc,dgquintas/grpc,ejona86/grpc,tengyifei/grpc,yugui/grpc,zhimingxie/grpc,jboeuf/grpc,a-veitch/grpc,quizlet/grpc,podsvirov/grpc