close websocket handle in client

[StoicDeveloper]

Ideally, the websocket could be closed both explicitly, with a client.close() method, and implicitly when the client goes out of scope.
The current implementation causes an issue in jest integration tests where a test that uses WebsocketTransport will hang forever, or until the websocket times out, and there is no way to close the socket, since the client.transport.ws field is private.

[StoicDeveloper]

If you think this is a good idea, I'd be happy to submit a PR