I want to slightly change default expressjs behaviour of `res.json(obj)` method. I am trying to override it in my own middleware, the thing is I need to call its original inside. 

But now it just calls itself causing a stack overflow.

```
app.use(function(req, res, next) {
    res.json = function(obj) {
        function delete_null_properties(obj) {
            // ...
        }
        delete_null_properties(obj);

        res.json(obj);
    };
    next();
});

```