Update flask_classy.py

Move the automatic response making code to the end of the view function proxy. This allows for more complex before/after processing. A hypothetical example:

```python
class MyResource(FlaskView):
    serializers = {
        'index': serialize_collection,
        'default': serialize_resource,
    }

    def after_request(self, name, response):
        serializer = self.serializers.get(name, self.serializers['default'])
        return jsonify(serializer(response))

    def index(self):
        return Model.get_all()

    def post(self):
        return Model()

    def get(self, resource_id):
        return Model.get(resource_id)

    def put(self, resource_id):
        return Model.get(resource_id)
```