title: Django REST Framework: raise error when extra fields are present on POST
body: <p>When you're writing a serializer, it is trivial to specify which fields will be included (via <code>Meta</code>'s <code>fields</code>), setting read/write permissions on them and validating them. However, I was wondering if there is an easy way to specify that <em>only</em> the fields that are included are to be expected and any extra keys passed in should raise an error.</p>
  
  <p>E.g. say, I have a serializer</p>
  
  <pre><code>class ModelASerializer(serializers.ModelSerializer):
  
      class Meta:
          model = models.ModelA
          fields = ('name', 'number')
  </code></pre>
  
  <p>Supposed further that I don't have any validations.</p>
  
  <p>And I'm <code>POST</code>ing this payload to create a new instance</p>
  
  <pre><code>{
      "name": "test",
      "number": 5
  }
  </code></pre>
  
  <p>Everything is fine.</p>
  
  <p>But suppose my API has changed and now I'm also storing a new field, <code>title</code>, but forget to update my serializer. Clients will be sending payloads that look like</p>
  
  <pre><code>{
      "name": "test",
      "number": 5,
      "title": "Mr"
  }
  </code></pre>
  
  <p>However the serializer will simply ignore the extra key, not raising an exception.</p>
  
  <p>So, my question is: is there any way to make a serializer <em>only</em> expect fields specified in <code>fields</code> or -- if it's not a <code>ModelSerializer</code> -- fields, specified as members, and raise an error if it's not the case?</p>