raise ResponseError if an empty list is passed to SADD

Redis SADD will raise an error if an empty list is passed to it

![screenshot 2014-06-04 12 29 29](https://cloud.githubusercontent.com/assets/1268088/3171865/b691f808-ebcc-11e3-934a-e8b27b705e23.png)



(I really hope I got the flow this time. sorry for messing up in previous PRs)