Problem implementing werkzeug converters

I tried to adapt http://flask.pocoo.org/snippets/106/ to flask_classy.

But an exception is raised  when implementing the werkzeug converter. (TypeError: 'ObjectId' object is not callable)

```
class CView(flask_classy.FlaskView):
    @flask_classy.route('/<objectid:c_id>')
    def get(self, c_id):
        return c_id
```
