def unsubscribe(channel)
      @sub_callbacks.delete(channel)
      @subs.delete(channel)
      raw_send_command(:unsubscribe, [channel])
      return pubsub_deferrable(channel)
    end
def unsubscribe(channel)
      @sub_callbacks.delete(channel)
      @subs.delete(channel)
      raw_send_command(:unsubscribe, [channel])
      return pubsub_deferrable(channel)
    end
Unsubscribe all callbacks for a given channel

 @return [Deferrable] Redis unsubscribe call