Honor HTTP_X_FORWARDED_PROTO for Gravatar

When behind a proxy/load balancer and HTTP_X_FORWARDED_PROTO indicates
that MantisBT is accessed via https, make sure all resources are loaded via https.

Fixes #22689

core/http_api.php

@@ -40,7 +40,15 @@
  * @return boolean True if protocol is HTTPS
  */
 function http_is_protocol_https() {
-	return !empty( $_SERVER['HTTPS'] ) && ( utf8_strtolower( $_SERVER['HTTPS'] ) != 'off' );
+	if( isset( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) ) {
+		return strtolower( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) == 'https';
+	}
+
+	if( !empty( $_SERVER['HTTPS'] ) && ( strtolower( $_SERVER['HTTPS'] ) != 'off' ) ) {
+		return true;
+	}
+
+	return false;
 }
 
 /**