def ping(payload, &blk)
      send(type: :ping, stream: 0, payload: payload)
      once(:ack, &blk) if blk
    end
def ping(payload, &blk)
      send(type: :ping, stream: 0, payload: payload)
      once(:ack, &blk) if blk
    end
Sends PING frame to the peer.

 @param payload [String] optional payload must be 8 bytes long
 @param blk [Proc] callback to execute when PONG is received