Added tests for ZRANK.
Added tests for ZRANK.

channelgrabber/predis,RedisLabs/predis,johnhelmuth/predis,stokes3452/predis,WalterShe/predis,moria/predis,nguyenthaihan/predis,mrkeng/predis,gopalindians/predis,nrk/predis,RudyJessop/predis,oswaldderiemaecker/predis,zhangyancoder/predis,SwelenFrance/predis,quangnguyen90/predis,dzung2t/predis,protomouse/predis,CloudSide/predis,lvbaosong/predis,vend/predis,gencer/predis,SecureCloud-biz/predis,dominics/predis