title: Django Rest Framework - return Response(serializer.data) image field get null instead of the url
body: <p>I set an 'ImageField' in the model, using postman to post data. The image uploaded successfully. However, when my view returns serializers.data, the image field is null, which is supposed to be the URL of the image.  Does anyone have any ideas? I appreciate it so much.
  This is the response</p>
  
  <pre><code>{
      "user": 7,
      "category": "2018",
      "cover": null,
      "name": "Louisville"
  }
  </code></pre>
  
  <p>This is my model</p>
  
  <pre><code>class Journey(models.Model):
      def user_directory_path(self, filename):
          return 'media/journey_cover/{0}/{1}'.format(self.user_id, filename)
  
      user = models.ForeignKey(to=UserProfile, related_name="journeys", on_delete=models.CASCADE)
      category = models.CharField(null=False, max_length=50)
      cover = models.ImageField(upload_to=user_directory_path)
      created = models.DateTimeField(auto_now_add=True)
      name = models.CharField(null=False, max_length=50)
  </code></pre>
  
  <p>My serializer:</p>
  
  <pre><code>class JourneySerializer(serializers.ModelSerializer):
      user = serializers.PrimaryKeyRelatedField(queryset=UserProfile.objects.all())
      cover = serializers.ImageField(use_url=True)
  
      class Meta:
          model = Journey
          fields = ['user', 'category', 'cover', 'name']
  </code></pre>
  
  <p>My View</p>
  
  <pre><code>class JourneyList(APIView):
      permission_classes = [IsOwnerOrReadOnly, permissions.IsAuthenticatedOrReadOnly]
      parser_classes = [MultiPartParser, FormParser]
  
      def post(self, request):
          print(request.content_type)
          serializer = JourneySerializer(data=request.data)
          if serializer.is_valid(raise_exception=True):
              journey = serializer.create(serializer.validated_data)
              journey.save()
              return Response(serializer.data, status=status.HTTP_201_CREATED)
          return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
  </code></pre>