add X-Requested-With header

[fhahn]

JQuery and most other Javascript frameworks add the X-Requested-With="'XMLHttpRequest" header to each Ajax request [1] , so it's possible to check if the request was made via Ajax on the server side.

[1] https://github.com/jquery/jquery/blob/master/src/ajax/xhr.js#L92

[mythz]

Ok this is now pushed in trunk and deployed to: http://www.servicestack.net/jqbuilder/
Sorry for the delay, tho feature requests added via pull-requests are more expediently handled ;)