Fix RedisConnection to properly cache null objects

Hello @ArtemTitoulenko, 

Please review the following commits I made in branch 'xiao-redis-return-null'.

fd7b10e3b151dcb6e0967a013456a63c06ccaea5 (2013-08-12 23:25:40 -0700)
Fix RedisConnection to properly cache null objects


R=@ArtemTitoulenko