Add support for unsolicited PONG

According to the RFC6455 section 5.5.3 " A Pong frame MAY be sent unsolicited. This serves as a unidirectional heartbeat. A response to an unsolicited Pong frame is not expected."

In my use case I have a client who will sent this PONG now and then to indicate it is still alive. We are not using the ping/pong as we don't have a need for the response AND we want to minimize network traffic. as much as possible.