Rack 1.2's request.url method is buggy, as it gets the host part from
HTTP_X_FORWARDED_HOST, but the port from SERVER_PORT (which should be
used in conjuction with SERVER_ADDR).

This way, if the app is run behind a reverse proxy, it will create a
URL with <public_host>:<private port>, and pass this to Facebook,
so Facebook will redirect back to the URL with the wrong port.

Upgrading Rack requires upgrade to Rails 3.1.

This temporary solution configures OmniAuth using a copy of the newer
version of Rack::Request.