title: Django user get_all_permissions() is empty while user_permissions is set
body: <p>I added some permissions to a user via the admin interface.</p>
  
  <p>From some reason all the perm functions fail, e.g</p>
  
  <pre><code>&gt;&gt;&gt; user.get_all_permissions()
  set([])
  </code></pre>
  
  <p>But accessing the table directly, works:</p>
  
  <pre><code>&gt;&gt;&gt; user.user_permissions.all()
  (list of permissions as expected)
  </code></pre>
  
  <p>What can cause the "get_all_permissions" (and all the perm functions like has_perm()) to fail ?</p>
  
  <p>Thanks</p>