title: django-rest-framework: Cannot call `.is_valid()` as no `data=` keyword argument was passed when instantiating the serializer instance
body: <p>I've the following model:</p>
  
  <pre><code>class NoteCategory(models.Model):
      title = models.CharField(max_length=100, unique=True)
  
      def __unicode__(self):
          return '{}'.format(self.title)
  
  class PatientNote(models.Model):
      category = models.ForeignKey(NoteCategory)
      patient = models.ForeignKey(Patient)
      description = models.CharField(max_length=500)
      created_at = models.DateTimeField(auto_now_add=True)
  
      def __unicode__(self):
          return '{}'.format(self.description)
  </code></pre>
  
  <p>And the following serializer:</p>
  
  <pre><code>class PatientNoteSerializer(serializers.ModelSerializer):
      class Meta:
          model = PatientNote
  </code></pre>
  
  <p>I just want to make a POST on the PatientNote. The GET works and also the POST on other models works properly:</p>
  
  <pre><code>class PatientNoteViewSet(APIView):
      queryset = PatientNote.objects.all()
  
  
      def post(self, request, format=None):
          if not request.auth:
              return Response({})
          token = Token.objects.filter(key=request.auth)[0]
          user = token.user
  
          serializer = PatientNoteSerializer(request.data)
          if serializer.is_valid():
              serializer.save()
              return Response(serializer.data, status=status.HTTP_201_CREATED)
          return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
  </code></pre>
  
  <p>The request.data is a QueryDict, i.e.,</p>
  
  <pre><code>&lt;QueryDict: {u'category': [u'1'], u'patient': [u'1'], u'description': [u'da rest']}&gt;
  </code></pre>
  
  <p>It would be able to populate the two FKs, patient and category, through their IDs and the description is a simple text.</p>
  
  <p>The POST request is the following one (that works with other models):
  <img src="https://i.stack.imgur.com/DSDmu.png" alt="enter image description here"></p>
  
  <p>Anyway, the POST response is 500 with the following error:</p>
  
  <pre><code>AssertionError at /api/notes/
  </code></pre>
  
  <p>Cannot call <code>.is_valid()</code> as no <code>data=</code> keyword argument was passed when instantiating the serializer instance.</p>
  
  <p>The error is the same if I try to use it in the python shell.</p>