When under Redis#pipelined, some calls return nil values, which then cause errors in Redis objects. In particular, HashKeys#all gives a NoMethodError on nil. I've modified that method to properly handle the nil case, but there could be other places in which the change is necessary.
Please ignore this pull request if Redis objects do not support pipelining, or you feel that the Redis driver itself should return dummy, non-nil values.
Modified Redis#hgetall to work under pipelining