Handle X-Forwarded-Proto for reverse proxy setup.

Assumes that your reverse proxy will be sending the appropriate http header:

  X-Forwarded-Proto: https if the proxy is doing https for us (and passing http)
  X-Forwarded-Proto: http  if request is not https

http://tools.ietf.org/html/draft-ietf-appsawg-http-forwarded-10 section 5.4