Fix `sdiff` removing elements from the source set

The redis sdiff operation doesn't modify the first set or all the successive sets.
Fix `zunionstore` and `zinterstore` commands to work with normal sets

The native redis zunionstore and zinterstore commands can handle normal sets besides
sorted sets as input.
Merge branch 'ckrybus-fix-sdiff'

* ckrybus-fix-sdiff:
  Fix `sdiff` removing elements from the source set
Merge branch 'ckrybus-fix-zaggregate'

* ckrybus-fix-zaggregate:
  Fix `zunionstore` and `zinterstore` commands to work with normal sets