pass proper arguments to method callbacks

[nlevitt]

There is a regression in 0.49.0 introduced by #442 cc @bashlakov. If you have callbacks (on_open etc) that are methods on a class, but the class is not a subclass of WebsocketApp, it tries to call them with the wrong arguments, and you get an error like this:

2018-08-15 14:43:49,328 79988 ERROR WebsockThread:9222 websocket.error(_logging.py:61) error from callback <bound method WebsockReceiverThread._on_open of <WebsockReceiverThread(WebsockThread:9222, started daemon 123145314500608)>>: _on_open() missing 1 required positional argument: 'websock'