Missing / in the "destination" param when redirecting to /user/login

When an user tries to initiate the authorization flow (/oauth2/authorize) and no use session is currently active on the Drupal instance, she's redirected to /user/login, with a destination parameter pointing back to "oauth2/authorize".

Drupal 8 route paths must begin with slash, which is missing here, basically triggering a redirection error in some cases.