Client socket doesn't have hasListeners method

[Pringels]

Great work on this library. It saved my skin when I discovered that https://miragejs.com/ doesn't support websockets yet :)

This Issue is similar in nature to #170

According to the socket.io docs, client sockets should be able to query if a given event has actively registered listeners:

The socket actually inherits every method of the Emitter class, like hasListeners, once or off (to remove an event listener).

[Pringels]

Thank you!