mod_http_oauth2: Only add nonce when issuing a client_secret Not as important that the client_id be unique if there's no client_secret since the point was to make each issued client_secret distinct.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).