Allow closing WSMessageHandler stream with a close code

[liautaud]

Hi! First of all thanks for the library, it's been really great to use.

Would you be open to adding a way for a WSMessageHandler to close the server stream with .close(code: closeCode) frames in some way or another? The typical use case is closing a WebSocket connection after some unrecoverable error while wanting the client-side to know what the error actually is–in JavaScript, for instance, you can read event.code on a WebSocket "close" event.

I could already implement this behavior by falling back to the more cumbersome WSHandler level, but it seems like a frequent enough use case that it would make sense at the WSMessageHandler level too.