mod_http_oauth2: Reflect ALL attributes of the client registration Per RFC 7591: " Additionally, the authorization server MUST return all registered metadata about this client, including any fields provisioned by the authorization server itself. " The idea is that the server may replace/drop fields in the registration, so what gets reflected back to the client is the source of truth about the registration.

  * Make groups optional
  * Make groups work with both posixGroup and groupOfNames