title: DRF: JSONField in serializers with TextField in models cause stringification
body: <p>I am using python 2.7.11</p>
  
  <p>A have a model let's say <code>Game</code> that has a TextField that's supposed to store json values. TextField was chosen because the database is shared with hibernate ORM that doesn't support postgres JSONb natively. Thus I have:</p>
  
  <p>models.py:</p>
  
  <pre><code>@python_2_unicode_compatible
  class Game(models.Model):
      settings = models.TextField(default='{}')
  </code></pre>
  
  <p>serializers.py:</p>
  
  <pre><code>class GameSerializer(serializers.ModelSerializer):
      settings = serializers.JSONField()
  </code></pre>
  
  <p>Is there a clean way to handle this, having valid json strings in the database and returning them as json objects through the API?</p>