title: serializer.is_valid() failing though `required=False` - Django REST Framework
body: <p>I have a serializer like so:</p>
  
  <pre><code>class DataSetColumnSerializer(serializers.ModelSerializer):
      custom_target = serializers.PrimaryKeyRelatedField(required=False)
  
      class Meta:
          model = dataset_models.DataSetColumn
  </code></pre>
  
  <p>Yet: </p>
  
  <pre><code>ipdb&gt; columns[0]
  {u'display_name': u'guid', u'name': u'guid', u'data_type': u'STRING', u'custom_target': None, u'ignore': False, u'is_identifier': False, u'order': 1}
  ipdb&gt; serializer.is_valid()
  False
  ipdb&gt; serializer.errors
  [{'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}, {'custom_target': [u'This field cannot be blank.']}]
  ipdb&gt; serializer.fields['custom_target'].required
  False
  </code></pre>
  
  <p>What's going on?</p>