Error on site with a base path doing oauth authorize

Any site with a base path, like http://example.com/drupal/folder, gets this error when trying to do the oauth authorization:

Redirects to external URLs are not allowed by default, use \Drupal\Core\Routing\TrustedRedirectResponse for it.

This is being caused by the fix for issue 3087181 which added a slash to the start of the destination. It should probably be getting the URL from the oauth2_server.authorize route or using base_path() instead of a slash.