title: Foxx oauth2 configuration
body: <p>To play with the demo-sessions app, I've got the oauth2 app mounted on /oauth2 as required.
  In arangodb/Foxx doc, the oauth2 endpoints seems to be defined as strings (i.e <a href="https://github.com/arangodb-foxx/util-oauth2" rel="nofollow">https://github.com/arangodb-foxx/util-oauth2</a> )</p>
  
  <p>But when I perform that with correct urls, and try to play with oauth, I've got an error :
  ...\oauth2\APP\manifest.json\": attribute child \"authEndpoint\" fails because [\"authEndpoint\" must be an object] (was \"[object Object]\").]","... </p>
  
  <p>Oauth endpoints definitions are expected to be objects, not strings. 
  So what is the correct configuration for Foxx oauth2 ? 
  Thanks for help,</p>