Make WebSocketHttpExchange.getSession() return something useful

It doesn't currently seem to be used by anything, but I have a use for it. How would you feel about adding a boolean param like for a servlet's request that determines whether the session is created or not? Seems like the AsyncWebSocketHttpServerExchange could then check for a SessionManager attachment and return null if not there. Otherwise, it could call Sessions.getSession or getOrCreateSession depending on the passed boolean, using its HttpServerExchange member.

If that makes sense, I can work up a PR.