Don't send websocket subprotocol

Some time recently, Google Chrome started actually caring what the
client sends for this and what the server replies with. httpuv
doesn't currently have any logic for subprotocol selection, so
it always replies with no Sec-WebSocket-Protocol header, which
now Google Chrome reacts to by closing the websocket connection.

This problem goes away if we just don't send a subprotocol at all.