title: Using Super create method for ModelSerializer
body: <p>I am trying to save some work by using the create method of the ModelSerializer in an extension of the class and then adding in the extra fields that I need in the extension. When I do this however I get an error from DRF about not supporting writable fields in nested serializers. Is there some way to implement this so that I don't have to explicitly define each field in the create method and instead push that work onto the super constructor? Included is my code:</p>
  
  <pre><code>class CreateUserSerializer(ModelSerializer):
      school = SchoolSerializer(required=False) 
  
      class Meta:
          model = User
          fields = ('id', 'username', 'password', 'first_name', 'last_name',
                'user_type', 'school', 'email')
          extra_kwargs = {
              'password': {'write_only': True},
              'user_type': {'read_only': True}
          }
  
      def create(self, validated_data):
          original_validated_data = validated_data.copy()
          if 'password' in validated_data:
              password = validated_data.pop('password')
  
          user = super(CreateUserSerializer, self).create(validated_data)
          if 'password' in original_validated_data:
              user.set_password(original_validated_data['password'])
          if 'school' in original_validated_data:
              user.user_type = User.TYPE_ADVISOR
  
          return user
  </code></pre>
  
  <p>And this is the error that I am getting:</p>
  
  <pre><code>File "/serializers/user.py", line 41, in create
    user = super(CreateUserSerializer, self).create(validated_data)
  File "/lib/python2.7/site-packages/rest_framework/serializers.py", line 832, in create
    raise_errors_on_nested_writes('create', self, validated_data)
  File "/lib/python2.7/site-packages/rest_framework/serializers.py", line 724, in raise_errors_on_nested_writes
    class_name=serializer.__class__.__name__
  AssertionError: The `.create()` method does not support writable nestedfields by default.
  Write an explicit `.create()` method for serializer `api.serializers.user.CreateUserSerializer`, or set `read_only=True` on nested serializer fields.
  </code></pre>
  
  <p>I am using Rest Framework V3.3.1</p>