title: Django Rest Framework returns null id when overriding ListSerializer create method
body: <p>I'm experimenting the issue described on <a href="https://github.com/tomchristie/django-rest-framework/issues/1116" rel="nofollow">https://github.com/tomchristie/django-rest-framework/issues/1116</a>.</p>
  
  <p>I have a ListSerializer which changes some of the fields of the incoming formData ... I needed this because I have to extract the width and height for every image being uploaded.
  Prior to do this I tried overriding save() method on my model but it did not work...</p>
  
  <pre><code>class ImageUrl(models.Model):
      file = models.ImageField(upload_to="%Y/%m/%d")
      createdAt = models.DateTimeField(default=datetime.now, editable=False)
      modifiedAt = models.DateTimeField(default=datetime.now)
      width = models.PositiveIntegerField(editable=False, default=0)
      height = models.PositiveIntegerField(editable=False, default=0)
      description = models.TextField(blank=True, default="")
  
      class Meta:
          ordering = ('createdAt',)
  
      def save(self, *args, **kwargs):
          # If the obj already exists update its modifiedAt attr
          if self.id:
              #time_format = "%a %b %d %H:%M:%S %Y"
              #today =
              #self.modifiedAt = today.strftime(time_format)
              self.modifiedAt = datetime.datetime.today()
          super(ImageUrl, self).save(*args, **kwargs)
  
  
  class ImageUrlListSerializer(serializers.ListSerializer):
  
      def to_internal_value(self, data):
          ret = []
          errors = []
          #import pdb; pdb.set_trace()
          for item in data.getlist("file"):
              aux_item = {"file": item}
              try:
                  validated = self.child.run_validation(aux_item)
              except ValidationError as exc:
                  errors.append(exc.detail)
              else:
                  ret.append(validated)
                  errors.append({})
          if any(errors):
              raise ValidationError(errors)
  
          return ret
  
      def create(self, validated_data):
          #print "Validated data: ", validated_data
          images = [ImageUrl(**item) for item in validated_data]
          for img in images:
              self._set_image_attributes(image=img)
          saved_images = ImageUrl.objects.bulk_create(images)
          return saved_images
  
      def _set_image_attributes(self, image):
          ''' On save, update timestamps '''
          #time_format = "%a %b %d %H:%M:%S %Y"
          if not image.id:
              image.createdAt = datetime.datetime.today()
              pil_image = PIL.Image.open(image.file)
              width, height = pil_image.size
              image.width = width
              image.height = height
          image.modifiedAt = datetime.datetime.today()
  </code></pre>