Fix WebSocketClientConnection parameter

[TnTomato]

Hi,

This is my first time making PR to this repo. Wish this will help.

While developing libs based on Tornado, I found a mutable default argument value in WebSocketClientConnection.__init__.

subprotocols: Optional[List[str]] = []

I think it's better to set the default argument value to None to prevent from some unpredictable problems.

[bdarnell]

Thanks!