Make MockRedis#pipelined yield itself

As Redis#piplined does:
https://github.com/redis/redis-rb/blob/v3.0.4/lib/redis.rb#L2030

Change-Id: Ia1b4c665d4549407ff4e1b1bd413b1d6529af66c
Reviewed-on: https://gerrit.causes.com/24220
Tested-by: Lann Martin <lann@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>