Fix OBJECT arity

Previously, the command definition for the OBJECT command specified
a minimum of two args (and that it was variadic), which meant that
if you sent this:

OBJECT foo

When cluster was enabled, it would result in an assertion/SEGFAULT
when Redis was attempting to extract keys.

It appears that OBJECT is not variadic, and only ever takes 3 args.

https://gist.github.com/michael-grunder/25960ce1508396d0d36a
Merge pull request #1870 from michael-grunder/object_arity

Fix OBJECT arity
Merge remote-tracking branch 'upstream/unstable' into unstable
Redis 主从复制分析
Merge branch 'unstable' of github.com:xm-king/redis3-research into unstable
Redis 主从复制分析