Feature request: ability to set settings per response

I'd like to implement middleware that returns formatted json in the response when `pretty_ouput` is present on the query. I attempted this with the following code:

```js
// This code does not work as expected.
app.use(function(req, res, next) {
// Querystring `pretty_output` should prettify json output.
if (req.query['pretty_ouput']) {
app.set('json spaces', 2); // I'd like to set this only for this one request.
}
next();
});
```

However, subsequent requests without the `pretty_ouput` querystring are also formatted.

Is there a way to set the setting `json spaces` for only one request? Ideally I could call `res.set('json spaces', 2)` that would set this option for only this one response. I'd think this would be generically useful for any Express option.