title: ImageSpecField simply does nothing with django-imagekit in django-rest-framework
body: <p>I've got the following setup:</p>
  
  <pre><code>#models.py
  class Image(models.Model):
      original = models.ImageField(upload_to='images/')
      formatted_image = ImageSpecField(source='original', format='JPEG', options={'quality': 90})
  
  
  #serializers.py
  class ImageSerializer(serializers.ModelSerializer):
      class Meta:
          model = Image
  
  #views.py
  class ImageViewSet(viewsets.ModelViewSet):
      model = Image
      serializer_class = ImageSerializer
  </code></pre>
  
  <p>According to the ImageKit documentation this should do the trick but literally nothing happens. Not even an error or anything. What am I doing wrong?</p>