mod_http_oauth2: Separate extracting credentials from requests and verifying The token endpoint also uses Basic auth, but the password would be the client_secret, so we need to verify against that instead of using test_password(). Splitting this up here avoids code duplication. Possibly this new function could go into util.http...

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).