client: fix TestSimpleHTTPClientDoCancelContextResponseBodyClosed
client: fix TestSimpleHTTPClientDoCancelContextResponseBodyClosed

This fixes the bug that the test may hang forever because RoundTrip is
blocked. fixes #2449

wantonsolutions/dinv-etcd,luxas/etcd,devfreesg/etcd,kkaneda/etcd,HideoYamauchi/etcd,steveeJ/etcd,xebitstudios/etcd,improbable-io/etcd,Eagle-X/etcd,ajityagaty/etcd,spacejam/etcd,luxas/etcd,fgbreel/etcd,freeloop1114/etcd,hongchaodeng/etcd,jlamillan/etcd,zaphod-concur/etcd,jefby/etcd,CrazyJvm/etcd,insequent/etcd,raoofm/etcd,cockroachdb/etcd,cchamplin/etcd,jefby/etcd,gouyang/etcd,tamird/etcd,fasaxc/etcd,cognusion/etcd,beni55/etcd,gae123/etcd,buaazp/etcd,giantswarm/etcd,etcd-io/etcd,fasaxc/etcd,spacejam/etcd,svdata/etcd,StorReduce/etcd,nekto0n/etcd,linuxcer/etcd,snailwalker/etcd,shakefu/etcd,Timer/etcd,chenbaihu/etcd,fgbreel/etcd,combk8s/etcd,nekto0n/etcd,mitake/etcd,sofuture/etcd,rchicoli/etcd,jravetch/etcd,honeyqa/etcd,ravigadde/etcd,YuleiXiao/etcd,nkhare/etcd,Yhgenomics/etcd,jmptrader/etcd,tangkun75/etcd,raoofm/etcd,shafreeck/etcd,insionng/etcd,YuleiXiao/etcd,QuantiumTechnology/etcd,dalanlan/etcd,glycerine/etcd,jchauncey/etcd,imzhulei/etcd,a-robinson/etcd,etcd-io/etcd,pengzj/etcd,armstrongli/etcd,a-robinson/etcd,gyuho/etcd,shawnps/etcd,es-chow/etcd,xiang90/etcd,princeqjzh/etcd,onlyjob/etcd,olark/etcd,handsomesun/etcd,bdarnell/etcd,swingbach/etcd,oscarrr110/etcd,wowsoso/etcd,gtank/etcd,mqliang/etcd,qiaochao911/etcd,insionng/etcd,njharman/etcd,li-ang/etcd,achanda/etcd,achanda/etcd,hzy001/etcd,yichengq/etcd,ktateish/etcd,glycerine/etcd,TheHippo/etcd,freeloop1114/etcd,dannysauer/etcd,wowsoso/etcd,betawaffle/etcd,honeyqa/etcd,shakefu/etcd,pauldallen/etcd,tamird/etcd,tguangch/etcd,glevand/coreos--etcd,whuwxl/etcd,tangkun75/etcd,imzhulei/etcd,jchauncey/etcd,Winslett/etcd,barakmich/etcd,HideoYamauchi/etcd,rnaveiras/etcd,hzy001/etcd,wfxiang08/etcd,cchamplin/etcd,offscale/etcd,msanterre/etcd,westhood/etcd,sebschrader/etcd,dnaeon/etcd,mkumatag/etcd,visheshnp/etcd,rnaveiras/etcd,purpleidea/etcd,lzbgt/etcd,sofuture/etcd,axingblog/etcd,purpleidea/etcd,openedbox/etcd,wfxiang08/etcd,coreos/etcd,xueshanf/etcd,junxu/etcd,OyTao/etcd_learning,TheHippo/etcd,chipironcin/etcd,xdlinear/etcd,hongbinz/etcd,devfreesg/etcd,mkumatag/etcd,xychu/etcd,glevand/coreos--etcd,fasaxc/etcd,mitake/etcd,dannysauer/etcd,sebschrader/etcd,shawnps/etcd,ylong/etcd,improbable-io/etcd,axingblog/etcd,QuantiumTechnology/etcd,sinsharat/etcd,swingbach/etcd,siddontang/etcd,gyuho/etcd,coreos/etcd,youxidev/etcd,mickep76/etcd,mqliang/etcd,ajityagaty/etcd,tempbottle/etcd,elimisteve/etcd,ktateish/etcd,linuxcer/etcd,lumjjb/etcd,akauppi/etcd,yichengq/etcd,polvi/etcd,Eagle-X/etcd,qqsun8819/etcd,vimalk78/etcd,iamqizhao/etcd,rnaveiras/etcd,barakmich/etcd,csuideal/etcd,steveeJ/etcd,jeremyeder/etcd,bluebreezecf/etcd,combk8s/etcd,chipironcin/etcd,elimisteve/etcd,xychu/etcd,sebrandon1/etcd,sayden/etcd,cognusion/etcd,junxu/etcd,betawaffle/etcd,pauldallen/etcd,xdlinear/etcd,polvi/etcd,lumjjb/etcd,visheshnp/etcd,sinsharat/etcd,Timer/etcd,hroyrh/etcd,lorneli/etcd,njucslqq/etcd,mickep76/etcd,bspline/etcd,onlyjob/etcd,svdata/etcd,siddontang/etcd,yudai/etcd,dnaeon/etcd,mike-lerch/etcd,plasticbox/etcd,SivagnanamCiena/etcd,arthurcolle/etcd,shafreeck/etcd,asim/etcd,insequent/etcd,tguangch/etcd,endocode/etcd,qiaochao911/etcd,lorneli/etcd,lzbgt/etcd,kingland/etcd,mkumatag/etcd,kkaneda/etcd,jmptrader/etcd,rchicoli/etcd,delkyd/etcd,arthurcolle/etcd,heyitsanthony/etcd,whuwxl/etcd,Winslett/etcd,oscarrr110/etcd,gityf/etcd,wantonsolutions/dinv-etcd,jonboulle/etcd,delkyd/etcd,es-chow/etcd,hongchaodeng/etcd,ylong/etcd,asim/etcd,plasticbox/etcd,hongbinz/etcd,westhood/etcd,princeqjzh/etcd,sebrandon1/etcd,yyljlyy/etcd,wantonsolutions/dinv-etcd,StorReduce/etcd,jonboulle/etcd,msanterre/etcd,chenbaihu/etcd,hroyrh/etcd,xueshanf/etcd,njucslqq/etcd,beni55/etcd,sayden/etcd,bspline/etcd,fanminshi/etcd,yyljlyy/etcd,pengzj/etcd,gae123/etcd,handsomesun/etcd,bluebreezecf/etcd,armstrongli/etcd,jlamillan/etcd,zaphod-concur/etcd,ajityagaty/etcd,SivagnanamCiena/etcd,csuideal/etcd,cockroachdb/etcd,qqsun8819/etcd,ravigadde/etcd,nkhare/etcd,fanminshi/etcd,dalanlan/etcd,kingland/etcd,akauppi/etcd,Yhgenomics/etcd,crandles/etcd,mike-lerch/etcd,li-ang/etcd,CrazyJvm/etcd,jeremyeder/etcd,openedbox/etcd,joshix/etcd,giantswarm/etcd,buaazp/etcd,youxidev/etcd,gtank/etcd,etcd-io/etcd,xiang90/etcd,crandles/etcd,heyitsanthony/etcd,snailwalker/etcd,bdarnell/etcd,jravetch/etcd,offscale/etcd,iamqizhao/etcd,joshix/etcd,endocode/etcd,vimalk78/etcd,yudai/etcd,mqliang/etcd,tempbottle/etcd,gouyang/etcd,njharman/etcd,xebitstudios/etcd,gityf/etcd,OyTao/etcd_learning,olark/etcd