Spec up #zinterstore behaviour
Spec up #zinterstore behaviour

Mostly examples taken from the redis-rb sorted spec test/unit file. RSpecs passing with redis-rb v3.0.1, failing with fakeredis.

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