MGET must never fail and return (nil) on non string entry.

expected:

     127.0.0.1:6379> sadd onset val1
     (integer) 1
     127.0.0.1:6379> mget onset
     1) (nil)

actual:

     127.0.0.1:6379> sadd onset val1
     (integer) 1
     127.0.0.1:6379> mget onset
     (error) ERRWRONGTYPE Operation against a key holding the wrong kind of value