Add Reply-To header even if it's the same as the Return-Path header

I'm trying to add a Reply-To header in a hook_mail_alter(), like so:

/**
 * Implements hook_mail_alter().
 */
function my_module_mail_alter(&$message) {
  $message['headers']['Reply-To'] = $message['from'];
  // $message['from'] = // something else here
}

However, in smtp.mail.inc it's only adding the Reply-To header if it differs from the Return-Path header. This causes problems later, because my SMTP connection is to SendGrid, which adds it's own Return-Path header. Thus, there's no way to add a Reply-To header.

For a good explanation of the difference between these two headers, see here:

https://stackoverflow.com/q/1235534

We should add it unconditionally, if it's defined.