Respect the header HTTP_X_FORWARDED_PROTO when the host doesn't include a port number.

Otherwise rack generates urls like https://example.com:80/foo/bar
Make sure that the default port is 80 when the request doesn't include HTTP_X_FORWARDED_PROTO.
Merge pull request #200 from calavera/respect_x_forwarded_proto

Respect the header HTTP_X_FORWARDED_PROTO.