title: aggregate(Max('id')) returns exception 'str' object has no attribute 'email'
body: <p>I've been trying to get the user with the highest id but no success.
  This is my user model:</p>
  
  <pre><code>class User(models.Model):
      email=models.EmailField(unique=True, null=False)
      name=models.TextField(null=True)
  </code></pre>
  
  <p>Its serializer:</p>
  
  <pre><code>class UserSerializer(serializers.ModelSerializer):
      class Meta:
          model = User
          fields = ('id', 'email', 'name')
  </code></pre>
  
  <p>The view:</p>
  
  <pre><code>class GetHighestValue(generics.ListAPIView):
      serializer_class = UserSerializer
  
      def get_queryset(self):
          return User.objects.aggregate(Max('id'))
  </code></pre>
  
  <blockquote>
    <p>Got AttributeError when attempting to get a value for field <code>email</code> on
    serializer <code>UserSerializer</code>. The serializer field might be named
    incorrectly and not match any attribute or key on the <code>str</code> instance.
    Original exception text was: 'str' object has no attribute 'email'.</p>
  </blockquote>
  
  <pre><code>Traceback (most recent call last):   
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 149, in get_response
       response = self.process_exception_by_middleware(e, request)   
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 147, in get_response
       response = wrapped_callback(request, *callback_args, **callback_kwargs)   
  File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
       return view_func(*args, **kwargs)   
  File "/usr/local/lib/python2.7/dist-packages/django/views/generic/base.py", line 68, in view
       return self.dispatch(request, *args, **kwargs)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/views.py", line 466, in dispatch
       response = self.handle_exception(exc)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/views.py", line 463, in dispatch
       response = handler(request, *args, **kwargs)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/generics.py", line 201, in get
       return self.list(request, *args, **kwargs)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/mixins.py", line 48, in list
       return Response(serializer.data)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 674, in data
       ret = super(ListSerializer, self).data   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 239, in data
       self._data = self.to_representation(self.instance)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 614, in to_representation
       self.child.to_representation(item) for item in iterable   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 463, in to_representation
       attribute = field.get_attribute(instance)   
  File "/home/user/.local/lib/python2.7/site-packages/rest_framework/fields.py", line 422, in get_attribute
       raise type(exc)(msg) 
       AttributeError: Got AttributeError when attempting to get a value for field `email` on serializer `UserSerializer`. 
       The serializer field might be named incorrectly and not match any attribute or key on the `str` instance. 
       Original exception text was: 'str' object has no attribute 'email'.
  </code></pre>