fix: broken behavior for Pub/Sub

[supercaracal]

• Sharded pub/sub suport in RedisClient::Cluster::PubSub #214
• fix: client should be able to subscribe multiple channels for Pub/Sub #215
• client should return nil if messages are none
  • https://github.com/redis-rb/redis-client#publish--subscribe

[supercaracal]

#test_sharded_pubsub_with_multiple_channels

SEGV occured if using hiredis. Ruby prevents race condition by GIL. Global Pub/Sub works fine.