title: id field in django rest framework serializer
body: <p>I'm using django rest framework and just creating a simple serializer like this:</p>
  
  <pre><code>class PackageSerializer(serializers.HyperlinkedModelSerializer):
  
      class Meta:
          model = Package
          fields = ('id', 'url', 'title','location')
  </code></pre>
  
  <p>However I get this error:</p>
  
  <pre><code>KeyError at /cerberus/packages/
  'id'
  </code></pre>
  
  <p>How come I can't get the primary key 'id' as part of my serialized data? Thanks for helping!</p>