Add specs for X-Forwarded-Host request recognition.

This is a pull request to test for and verify tylerhunt/rack-canonical-host#14.

It appears as though [Rack should recognize](https://github.com/rack/rack/commit/fb32ff74f1e68650b7b4fa9889d277ef1fd30380) the `X-Forwarded-Host` header and transparently use it for the `Rack::Request#host` value (since Dec 2009). Since this library is using `Rack::Request` to parse the request environment and capture the `url`, it _should_ be properly recognized and correctly redirect or allow.

This pull request simply adds specs to verify and ensure the functionality.