title: Facebook Oauth access_token andpoint does not return "expires" value
body: <p>I have a strange problem with Facebook Oauth access_token endpoint:</p>
  
  <pre><code>https://graph.facebook.com/oauth/access_token?             
      client_id=APP_ID&amp;
      client_secret=APP_SECRET&amp;
      grant_type=fb_exchange_token&amp;
      fb_exchange_token=EXISTING_ACCESS_TOKEN
  </code></pre>
  
  <p>The documentation <a href="https://developers.facebook.com/roadmap/offline-access-removal/" rel="noreferrer">here</a> (in "Scenario 3") said:
  "If the call is made while there is still a valid long-lived user access_token for that user, the returned user access_token from this second call may be the same or may have changed, but in either case the expiration time will be set to a long expiration time."</p>
  
  <p>If I obtain a valid long-lived user access_token, this endpoint returns the same access token I already have and expires parameter too <strong>but only for a short period</strong>.
  So if I repeat the same call to the above andpoint with the same access_token after some time (few hours for example) it returns only the access_token without the expires parameter.
  ...so I can't establish when my access_token will expires.
  Note that the access_token returned is still valid and it works perfectly.</p>
  
  <p>Have you experienced the same issue?
  Do you have any suggestions?
  Thank you!</p>