title: Django REST Framework. Update gets an empty validated_data
body: <p>I made the serializer below. When I use POST for creating or GET for getting instance, then all right. But when I use PATCH method for updating an instance, then update method gets an empty dict as validated_data. What is reason of this? If I use a fundamentally wrong approach, please show me right way. Thanks!</p>
  
  <pre><code>class UserSerializer(serializers.ModelSerializer):
      # password = serializers.CharField(write_only=True)
  
      class Meta:
          model = get_user_model()
          fields = ('id', 'email', 'first_name', 'last_name', 'password', )
          read_only_fields = ('is_staff', 'is_superuser', 'is_active', )
          extra_kwargs = {'password': {'write_only': True}}
  
      def create(self, validated_data):
          password = validated_data.pop('password')
          user = self.Meta.model(**validated_data)
          user.set_password(password)
          user.is_active = True
          user.save()
          return user
  
      def update(self, instance, validated_data):
          print validated_data  # returns {} - empty dict
          return instance
  </code></pre>
  
  <p>I use ModelViewSet as view:</p>
  
  <pre><code>class UserViewSet(viewsets.ModelViewSet):
      queryset = get_user_model().objects.all()
      serializer_class = UserSerializer
  </code></pre>
  
  <p>urls.py:</p>
  
  <pre><code>router = routers.DefaultRouter()
  router.register(r'accounts', UserViewSet, base_name='User')
  
  urlpatterns = [
      url(r'', include(router.urls)),
  ]
  </code></pre>
  
  <p>For testing I use the Pycharm REST Client and send "first_name=Name" as request parameter for PATCH /accounts/1/
  The print on serializer above shows me "{}". Needed account is exist and the serializer receives the target instance.</p>