This change fixes several issues with WebSocket and proxy authentication. The AuthenticationFilter is changed to support an authenticating server accessed through an authenticating proxy. MultiExchange is fixed to close the previous connection if a new connection is necessary to establish the websocket (websocket connections are not cached and must be closed in that case). WebSocket OpeningHandshake is fixed to close the connection (without creating the RawChannel) if the opening handshake doesn't result in 101 upgrade protocol.
Reviewed-by: prappo, chegar
