Response header not being set with Malsup Jquery form plugin

Response header not being set with Malsup Jquery form plugin

I have a web application written mostly in xQuery that uses  Malsups Ajax form plugin  to submit a webform via an Ajax call. 

I'm having an issue in IE9 where the header  X-Requested-With: XMLHttpRequest is not included in the server request. The header is there when I try in Chrome, Firefox, and even IE10.

Is there a reason why this header doesn't get added in IE9 and is there a quick way I can manually add it?

Please see my post on StackOverflow here to see a code snippet and a more detailed description: