fix(proxy): get proxied socket.io websocket transport working.

This was motivated by long (10s) default timeouts when socket.io attempts to use non-working websockets.

See #532 and https://github.com/nodejitsu/node-http-proxy#proxying-websockets for more details.

I'm not sure this is completely correct, but it does get our e2e login test working via websocket transport.