title: ForeignKey does not allow null values
body: <p>I am using the Django REST Framework 2.0.</p>
  <p>Here is my model class:</p>
  <pre><code>class Mission(models.Model):
    assigned_to = models.ForeignKey('auth.User',
                                     related_name='missions_assigned',
                                     blank = True)
  </code></pre>
  <p>Here is my view class:</p>
  <pre><code>class MissionList(generics.ListCreateAPIView):
      model = Mission
      serialize_class = MissionSerializer
  </code></pre>
  <ol>
  <li><p>The multipart form is rendered in the browser with empty choice for <code>assigned_to</code> field.</p>
  </li>
  <li><p>When posting raw JSON, I get the following error message:</p>
  </li>
  </ol>
  <p><code>Cannot assign None: &quot;Mission.assigned_to&quot; does not allow null values.</code></p>