sendMessage does not check if passed input message is compatible with $this
WebSocketConnection type. As a result, if sendMessage is called for
WebSocketConnectionHixie with WebSocketMessage instance as input (not
WebSocketMessage76 as it is supposed) - than nothing happens - no error and no
sending data.
Should fire exception or handle such cases internaly by coverting input message
to proper type. Or better yet, WebSocketConnection should provide a method to
construct a message of proper type, and direct creation via
WebSocketMessage::create must be eliminated.