Downgrade xhr headers

There is currently an odd bug where `redirect_to somelocation` in a POST will not properly set the `X-XHR-Redirected-To`.

So the problem is, turbolinks had some X-XHR fixes on the ruby side. This fixed a CORS error, but now doesn't seem to be redirecting properly.  `X-XHR-Redirected-To` header is no longer being sent by the server after a POST with redirect_to some new location, when performed by an XHR.

This PR reverts the behaviour to send `X-XHR-Redirected-To` more often (the module `XHRHeaders` to version @ https://raw.githubusercontent.com/rails/turbolinks/v1.3.1/lib/turbolinks.rb)