Signature failures for drupal behind proxy SSL server

The library generates the url to use when checking the signature by looking at the $_SERVER['HTTPS'] variable.
That's not always correct when a proxy server is in play. We can also use the X-Forwarded-Proto header to determine scheme.

When a drupal site is behind a separate server that does SSL
termination, it can get confused about whether the signed URL
should be https or not. SSL termination or proxy servers add the
X-Forwarded-Proto header to indicate the original request's scheme.