title: Field Field name `test` is not valid for model `Ability`
body: <p>I have a model without field 'test'. I'm assigning this field in runtime:</p>
  
  <pre><code>ability = Ability.objects.first()
  ability.test = 'TEST!!'
  </code></pre>
  
  <p>I also have the serilizer:</p>
  
  <pre><code>class AbilitySerializer(serializers.ModelSerializer):
      class Meta:
          model = Ability
          fields = ('name', 'test',)
  </code></pre>
  
  <p>And when I use it:</p>
  
  <pre><code>return Response(AbilitySerializer(ability).data)
  </code></pre>
  
  <p>I'm getting error:</p>
  
  <pre><code>Field Field name `test` is not valid for model `Ability`.
  </code></pre>
  
  <p><strong>EDIT</strong>: I'm still facing this issue when I'm passing array of objects to serializer (with many=True). It's OK when I pass single instance.</p>
  
  <p>Why and how to fix it?</p>