WebSocket xmpp sub protocol support

Starting with version 30, Chrome throw an exception if there is a sub-protocol mismatch:
> WebSocket connection to 'XXX' failed: Error during WebSocket handshake: Sec-WebSocket-Protocol mismatch lightstring.js:927

This patch allows connections with undefined sub protocol and xmpp sub protocol.
If the xmpp sub protocol was specified, it is sent back during the handshake.