Set X-Forwarded-Proto Header

Heroku passes the [X-Forwarded-Proto header][1] to indicate the protocol of
the incoming request. Ruby's rack [relies on this header][2] to
determine if a request is HTTPS.

Also, the X-Scheme header was removed to eliminate redundant passing of
the scheme.

[1]: https://devcenter.heroku.com/articles/http-routing#heroku-headers
[2]: https://github.com/rack/rack/blob/master/lib/rack/request.rb#L77