Better method of checking if request is XHR

While most AJAX frameworks have support for the X-Requested-With header, others are dropping it, citing low usage.

Angular:
https://github.com/angular/angular.js/commit/3a75b1124d062f64093a90b26630938558909e8d

I also believe this is a better way of determining what content to send back for building APIs, despite that it is not actually checking whether the request was XHR.