Fix -[RKObjectMapping mappingForSourceKeyPath:] when sourceKeyPath is nil
Fix -[RKObjectMapping mappingForSourceKeyPath:] when sourceKeyPath is nil

Closes https://github.com/RestKit/RestKit/issues/1937

pat2man/RestKit,fedegasp/RestKit,Bogon/RestKit,Gaantz/RestKit,fedegasp/RestKit,justinyaoqi/RestKit,oye-lionel/GithubTest,braindata/RestKit-1,ipmobiletech/RestKit,HarrisLee/RestKit,jrtaal/RestKit,DejaMi/RestKit,naqi/RestKit,pbogdanv/RestKit,nphkh/RestKit,jonesgithub/RestKit,apontador/RestKit,RestKit/RestKit,ChinaPicture/RestKit,goldstar/RestKit,wangjiangwen/RestKit,aleufms/RestKit,moneytree/RestKit,jonesgithub/RestKit,damiannz/RestKit,PonderProducts/RestKit,zhenlove/RestKit,Bogon/RestKit,pbogdanv/RestKit,vilinskiy-playdayteam/RestKit,kevmeyer/RestKit,SuPair/RestKit,pat2man/RestKit,gauravstomar/RestKit,pbogdanv/RestKit,samanalysis/RestKit,cryptojuice/RestKit,oligriffiths/RestKit,justinyaoqi/RestKit,ipmobiletech/RestKit,zhenlove/RestKit,braindata/RestKit-1,StasanTelnov/RestKit,Livestream/RestKit,oye-lionel/GithubTest,PonderProducts/RestKit,goldstar/RestKit,TheFarm/RestKit,common2015/RestKit,ppierson/RestKit,timbodeit/RestKit,samanalysis/RestKit,apontador/RestKit,QLGu/RestKit,hejunbinlan/RestKit,0x73/RestKit,zilaiyedaren/RestKit,zhenlove/RestKit,gank0326/restkit,oligriffiths/RestKit,Livestream/RestKit,adozenlines/RestKit,hejunbinlan/RestKit,aleufms/RestKit,loverbabyz/RestKit,lmirosevic/RestKit,naqi/RestKit,nphkh/RestKit,taptaptap/RestKit,samkrishna/RestKit,fhchina/RestKit,zilaiyedaren/RestKit,lmirosevic/RestKit,pat2man/RestKit,Juraldinio/RestKit,SuPair/RestKit,naqi/RestKit,kevmeyer/RestKit,oligriffiths/RestKit,REXLabsInc/RestKit,TheFarm/RestKit,antondarki/RestKit,cnbin/RestKit,damiannz/RestKit,vilinskiy-playdayteam/RestKit,caamorales/RestKit,jrtaal/RestKit,sachin-khard/NucleusRestKit,zjh171/RestKit,djz-code/RestKit,apontador/RestKit,lucasecf/RestKit,fhchina/RestKit,CodewareTechnology/RestKit,0x73/RestKit,sachin-khard/NucleusRestKit,canaydogan/RestKit,qingsong-xu/RestKit,kevmeyer/RestKit,coderChrisLee/RestKit,fedegasp/RestKit,common2015/RestKit,oye-lionel/GithubTest,fhchina/RestKit,canaydogan/RestKit,Papercloud/RestKit,Flutterbee/RestKit,nett55/RestKit,taptaptap/RestKit,baumatron/RestKit,PonderProducts/RestKit,youssman/RestKit,DejaMi/RestKit,Flutterbee/RestKit,moneytree/RestKit,Meihualu/RestKit,moneytree/RestKit,imton/RestKit,Gaantz/RestKit,lucasecf/RestKit,mavericksunny/RestKit,gauravstomar/RestKit,wangjiangwen/RestKit,hanangellove/RestKit,QLGu/RestKit,common2015/RestKit,canaydogan/RestKit,ppierson/RestKit,loverbabyz/RestKit,TheFarm/RestKit,HarrisLee/RestKit,imton/RestKit,sachin-khard/NucleusRestKit,ForrestAlfred/RestKit,coderChrisLee/RestKit,loverbabyz/RestKit,nett55/RestKit,Juraldinio/RestKit,ppierson/RestKit,nphkh/RestKit,antondarki/RestKit,taptaptap/RestKit,wuxsoft/RestKit,mumer92/RestKit,common2015/RestKit,hejunbinlan/RestKit,nphkh/RestKit,moneytree/RestKit,REXLabsInc/RestKit,dx285/RestKit,Papercloud/RestKit,mumer92/RestKit,QLGu/RestKit,qingsong-xu/RestKit,ForrestAlfred/RestKit,cryptojuice/RestKit,qingsong-xu/RestKit,wireitcollege/RestKit,money-alex2006hw/RestKit,SuPair/RestKit,ipmobiletech/RestKit,Flutterbee/RestKit,ForrestAlfred/RestKit,RestKit/RestKit,RyanCodes/RestKit,ChinaPicture/RestKit,DejaMi/RestKit,sachin-khard/NucleusRestKit,jrtaal/RestKit,coderChrisLee/RestKit,wangjiangwen/RestKit,braindata/RestKit-1,Livestream/RestKit,hanangellove/RestKit,lmirosevic/RestKit,fedegasp/RestKit,LiuShulong/RestKit,SuPair/RestKit,nphkh/RestKit,HarrisLee/RestKit,wuxsoft/RestKit,braindata/RestKit-1,mumer92/RestKit,Meihualu/RestKit,cnbin/RestKit,apontador/RestKit,Meihualu/RestKit,cryptojuice/RestKit,CodewareTechnology/RestKit,dx285/RestKit,hejunbinlan/RestKit,mberube09/RestKit,pbogdanv/RestKit,caamorales/RestKit,mumer92/RestKit,ipmobiletech/RestKit,adozenlines/RestKit,sachin-khard/NucRestKit,oye-lionel/GithubTest,Papercloud/RestKit,pbogdanv/RestKit,cryptojuice/RestKit,sachin-khard/NucRestKit,gauravstomar/RestKit,oye-lionel/GithubTest,Papercloud/RestKit,erichedstrom/RestKit,zhenlove/RestKit,vilinskiy-playdayteam/RestKit,cfis/RestKit,wuxsoft/RestKit,youssman/RestKit,qingsong-xu/RestKit,coderChrisLee/RestKit,imton/RestKit,0x73/RestKit,zjh171/RestKit,Juraldinio/RestKit,DejaMi/RestKit,justinyaoqi/RestKit,mberube09/RestKit,wireitcollege/RestKit,ipmobiletech/RestKit,samkrishna/RestKit,mavericksunny/RestKit,common2015/RestKit,samanalysis/RestKit,djz-code/RestKit,dx285/RestKit,Bogon/RestKit,nett55/RestKit,cryptojuice/RestKit,wireitcollege/RestKit,erichedstrom/RestKit,taptaptap/RestKit,Bogon/RestKit,antondarki/RestKit,aleufms/RestKit,fhchina/RestKit,adozenlines/RestKit,jrtaal/RestKit,Flutterbee/RestKit,Gaantz/RestKit,baumatron/RestKit,RyanCodes/RestKit,RyanCodes/RestKit,sandyway/RestKit,0x73/RestKit,zjh171/RestKit,LiuShulong/RestKit,Meihualu/RestKit,ppierson/RestKit,lucasecf/RestKit,sandyway/RestKit,Gaantz/RestKit,lmirosevic/RestKit,hejunbinlan/RestKit,cnbin/RestKit,pat2man/RestKit,cfis/RestKit,LiuShulong/RestKit,money-alex2006hw/RestKit,Bogon/RestKit,DejaMi/RestKit,CodewareTechnology/RestKit,QLGu/RestKit,caamorales/RestKit,hanangellove/RestKit,kevmeyer/RestKit,oligriffiths/RestKit,Gaantz/RestKit,Papercloud/RestKit,ChinaPicture/RestKit,goldstar/RestKit,lucasecf/RestKit,Juraldinio/RestKit,fedegasp/RestKit,aleufms/RestKit,mberube09/RestKit,wuxsoft/RestKit,sandyway/RestKit,oligriffiths/RestKit,REXLabsInc/RestKit,youssman/RestKit,loverbabyz/RestKit,ChinaPicture/RestKit,wireitcollege/RestKit,pat2man/RestKit,jrtaal/RestKit,Meihualu/RestKit,imton/RestKit,wangjiangwen/RestKit,ForrestAlfred/RestKit,sachin-khard/NucleusRestKit,justinyaoqi/RestKit,caamorales/RestKit,StasanTelnov/RestKit,cnbin/RestKit,imton/RestKit,gauravstomar/RestKit,Juraldinio/RestKit,Livestream/RestKit,adozenlines/RestKit,timbodeit/RestKit,zilaiyedaren/RestKit,zjh171/RestKit,qingsong-xu/RestKit,ChinaPicture/RestKit,jonesgithub/RestKit,gank0326/restkit,sandyway/RestKit,justinyaoqi/RestKit,lucasecf/RestKit,money-alex2006hw/RestKit,moneytree/RestKit,HarrisLee/RestKit,damiannz/RestKit,money-alex2006hw/RestKit,antondarki/RestKit,SuPair/RestKit,LiuShulong/RestKit,RyanCodes/RestKit,0x73/RestKit,nett55/RestKit,PonderProducts/RestKit,sandyway/RestKit,djz-code/RestKit,wuxsoft/RestKit,baumatron/RestKit,loverbabyz/RestKit,zhenlove/RestKit,mberube09/RestKit,vilinskiy-playdayteam/RestKit,coderChrisLee/RestKit,goldstar/RestKit,youssman/RestKit,mumer92/RestKit,gauravstomar/RestKit,dx285/RestKit,StasanTelnov/RestKit,CodewareTechnology/RestKit,money-alex2006hw/RestKit,naqi/RestKit,gank0326/restkit,mavericksunny/RestKit,gank0326/restkit,wangjiangwen/RestKit,QLGu/RestKit,canaydogan/RestKit,timbodeit/RestKit,ForrestAlfred/RestKit,cnbin/RestKit,erichedstrom/RestKit,zilaiyedaren/RestKit,damiannz/RestKit,hanangellove/RestKit,canaydogan/RestKit,baumatron/RestKit,gank0326/restkit,timbodeit/RestKit,fhchina/RestKit,damiannz/RestKit,djz-code/RestKit,dx285/RestKit,HarrisLee/RestKit,lmirosevic/RestKit,samanalysis/RestKit,nett55/RestKit,sachin-khard/NucRestKit,aleufms/RestKit,caamorales/RestKit,antondarki/RestKit,timbodeit/RestKit,jonesgithub/RestKit,sachin-khard/NucRestKit,ppierson/RestKit,sachin-khard/NucRestKit,hanangellove/RestKit,wireitcollege/RestKit,kevmeyer/RestKit,mavericksunny/RestKit,zilaiyedaren/RestKit,youssman/RestKit,adozenlines/RestKit,RestKit/RestKit,zjh171/RestKit,braindata/RestKit-1,cfis/RestKit,REXLabsInc/RestKit,LiuShulong/RestKit,samkrishna/RestKit,jonesgithub/RestKit,apontador/RestKit,CodewareTechnology/RestKit,samanalysis/RestKit,RyanCodes/RestKit,baumatron/RestKit