title: DRF Viewset custom "create" method does not call serializer custom "create" method
body: <p>referring to the code below. 
  What I'm trying to achieve is to create a new instance of "Survey" when an "Answer" is created.
  The POST request sent is: <code>{"choice":123}</code></p>
  
  <p>First problem:     "survey": [
          "This field is required."
      ]. Removing "survey" from the serializer fields gets around this but I'm not sure this is correct. Note that both "Survey" &amp; "Choice" have one-to-many relationship with "Answer" via foreign key.</p>
  
  <p>Second problem: The custom create method in the serializer is not called because the test print "HELLO" I put in is not executed and nothing is created.</p>
  
  <p>Been stuck on this for a while, any ideas guys? Much appreciate it.</p>
  
  <pre><code>class AddAnswerView(viewsets.ViewSet):
  
      def create(self, request, *args, **kwargs):
          data = request.data.get(
              "items") if 'items' in request.data else request.data
          many = isinstance(data, list)
          serializer = AnswerSerializer(data=data, many=many)
          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)
  
  class SurveySerializer(serializers.ModelSerializer):
  
      class Meta:
          model = models.Survey
          fields = ('id',)
  
      def create(self, validated_data):
          return models.Survey.objects.create(**validated_data)
  
  
  class AnswerSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = models.Answer
          fields = ('choice', 'survey')
  
      def create(self, validated_data):
          print('HELLO')
          user = self.context['request'].user
          new_survey = models.Survey.objects.create(user=user)
          answer = models.Answer.objects.create(survey=new_survey, 
            **validated_data)
          return answer
  </code></pre>