_fetchDataBundle(url) {
		return API.get(url, DATA_BUNDLE_OPTS).then(data => JSON.stringify(data.body));
	}
Adds a plugin function that can be used to modify the response
object before it is passed the caller's callback.

The callback function will take err and response as parameters
(like the callback to `end()`), and the request as well:
```
ReactServerAgent.plugResponse(function (err, response, request) {
    // e.g.
    console.log("Response received!", res.body);
    res.wasLogged = true; // or whatever
})
```