overload_xhr.js

var RealXMLHttpRequest = window.XMLHttpRequest;

window.XMLHttpRequest = function(){

    var asynchrony = new RealXMLHttpRequest();
    var realOpen = asynchrony.open;

    // steal the OPEN method | Has to be open to send header
    asynchrony.open = function(method, url){
        realOpen.apply(asynchrony, arguments);
        asynchrony.setRequestHeader('X-Custom', 'customtext');
    };
    return asynchrony;
};