title: Why is not `url` showing up in the DRF Response
body: <p>I can't get <code>url</code> to be returned when I use it in a <code>HyperlinkedModelSerializer</code>.</p>
  
  <pre class="lang-py prettyprint-override"><code># models.py
  class Promotion(TimeStampedMixin, models.Model):
      name = models.CharField(max_length=300)
      # ...
  
  
  # views.py
  class PromotionViewSet(viewsets.ModelViewSet):
      serializer_class = PromotionSerializer
      queryset = Promotion.objects.all()
  
      def retrieve(self, request, *args, **kwargs):
          instance = self.get_object()
          serializer = self.get_serializer(instance, context={'request': request})
          return Response(serializer.data)
  
  # serializers.py
  class PromotionSerializer(serializers.HyperlinkedModelSerializer):
      url = serializers.HyperlinkedRelatedField(
          view_name="campaigns:promotion-detail", read_only=True
      )
  
      class Meta:
          model = Promotion
          fields = (
              "url",
              "id",
              "name",
          )
  </code></pre>
  
  <p>The JSON output I receive when querying <code>curl -X GET http://localhost/api/promotion/2/</code>:</p>
  
  <pre class="lang-js prettyprint-override"><code>{"id":2,"name":"My promotion"}
  </code></pre>
  
  <p>If I use <code>reverse</code> to check if the <code>view_name</code> in the <code>HyperlinkedRelatedField</code> exists, it prints the correct URL.</p>
  
  <p>My question is: why doesn't the <code>url</code> show up in the response? It works any all my other views (comparing code with classes that works hasn't helped). Read the DRF documentation but I see nothing new (using version 3.11.0)</p>