Added tests for DISCARD.
Added tests for DISCARD.

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