title: ModelSerializer 'TypeError: "ReturnList" is not callable' with many=True
body: <p>I am trying to serialize a queryset from the model ObjectTag and then use it to serialize a reverse generic relationship in another model. However, this simple ModelSerializer is giving me problems when using many=True and a queryset or when serializing one instance.</p>
  
  <p>I have a model serializer as follows:</p>
  
  <pre><code>from rest_framework import serializers
  
  from . import models
  
  class ObjectTagSerializer(serializers.ModelSerializer):
      class Meta:
          model = models.ObjectTag
          fields = ('tag_content_type', 'object_id', 'object_content_type')
  </code></pre>
  
  <p>And the model is defined as follow:</p>
  
  <pre><code>class ObjectTag(models.Model):
      """
      Many to many to attach tags to objects.
      """
      object_content_type = models.ForeignKey(
          ContentType,
          related_name="object_object_tag" # necessary to avoid clash in ContentType
      )
      object_id = models.PositiveIntegerField()
      content_object = GenericForeignKey('object_content_type', 'object_id')
  
      tag_content_type = models.ForeignKey(
          ContentType,
          limit_choices_to={
              "model__in": tags.TagManager.tag_class_names,
              "app_label": tags.TagManager.app_label
          },
          related_name="tag_object_tag" # necessary to avoid clash in ContentType
      )
      tag_id = models.PositiveIntegerField()
      content_tag = GenericForeignKey('tag_content_type', 'tag_id')
  
      class Meta:
          unique_together = (
              ('tag_id', 'tag_content_type', 'object_id', 'object_content_type'),
          )
  
      def __str__(self):
          return u'object tag: {tag_type}: {tag} \u21D2 {obj}'.format(
              tag_type=self.content_tag._meta.verbose_name,
              tag=self.content_tag,
              obj=self.content_object,
          )
  </code></pre>
  
  <p>When I try to serialize a queryset with many=True from the model ObjectTag, I get the following error:</p>
  
  <pre><code>from .models import ObjectTag
  from .serializers import ObjectTagSerializer
  ot = ObjectTag.objects.filter(object_id=14691)
  ObjectTagSerializer(ot, many=True, read_only=True).data()
  Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
  TypeError: 'ReturnList' object is not callable
  </code></pre>
  
  <p>And when serializing one instance, I get the following:</p>
  
  <pre><code>ObjectTagSerializer(ot[0], read_only=True).data()
  Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
  TypeError: 'ReturnDict' object is not callable
  </code></pre>
  
  <p>I am using DRF 3.3.2 and Django 1.8 and Python 2.7.3</p>