Add HTTP_X_FORWARDED_PROTO support to OAuth

[seanfahey]

A tool provider behind a load balancer that passes requests internally over http will cause a "OAuth signature check failed" error. Check for a HTTP_X_FORWARDED_PROTO and HTTP_X_FORWARDED_PORT to build a correct request. Based off https://tracker.moodle.org/browse/MDL-49171

[joostdekeijzer]

Hi,

We're running into the same problem: LRS behind load balancer.

In our configuration the HTTP_X_FORWARDED_PORT isn't available, so we need something like:

    if ( isset($_SERVER['HTTP_X_FORWARDED_PROTO']) ) {
        $scheme = $_SERVER['HTTP_X_FORWARDED_PROTO'];

        if ( isset( $_SERVER['HTTP_X_FORWARDED_PORT'] ) ) {
            $port = $_SERVER['HTTP_X_FORWARDED_PORT'];
        } else if ( 'https' == $_SERVER['HTTP_X_FORWARDED_PROTO'] ) {
            $port = 443;
        }
    }

[renaatdemuynck]

This fixes my issues as well. Can this pull request be accepted?