title: Django REST Framework: Unique_together validation on Serializers
body: <p>Having issues with a <code>serializer.is_valid()</code> returning <code>True</code> if the serializer instance fails a <code>unique_together</code> constraint on the model side.</p>
  
  <p>Is there a way for me to specify in the serializer to enforce a <code>unique_together</code> constraint?</p>