title: Django Rest Framework use of `permissions.IsAuthenticated` rises an erro
body: <p>I have a <code>ModelViewSet</code> and I want it to be accessed only by authenticated users, so I added the <code>permission_classes = (permissions.IsAuthenticated)</code>, but I get an error: </p>
  
  <pre><code>TypeError at /es/general/countries/
  'type' object is not iterable
  </code></pre>
  
  <p>This is the <code>ViewSet</code>:</p>
  
  <pre><code>class CountryViewSet(viewsets.ModelViewSet):
      permission_classes = (permissions.IsAuthenticated)
      queryset = models.Country.objects.all()
      serializer_class = serializers.CountrySerializer
  </code></pre>
  
  <p>If I remove the <code>IsAuthenticated</code> assignment line, the ViewSet works, but with it, I get the error.</p>
  
  <p>I'll appreciate your help.</p>