Add specs for zunionstore command
Add specs for zunionstore command

hoffmanc/fakeredis,gotwalt/fakeredis,alienxp03/fakeredis,meetupcall/fakeredis,carsonreinke/fakeredis,zegal-cn/fakeredis,guilleiguaran/fakeredis,TiteiKo/fakeredis,alexlag/fakeredis,caulfield/fakeredis