fix xhr-polling response header problem.

Hi!

I added codes to change the response header based on the request body.
(node-socke.io-client fails connecting to go-engine.io server without this change)

The following is an implementation of the node version.
https://github.com/Automattic/engine.io/blob/master/lib/transports/polling-xhr.js#L60

Please merge if there is no problem.