How to test XMLHttpRequest with Jasmine

Question

How can I test the onreadystatechange on XMLHttpRequest or pure Javascript AJAX without jQuery? I'm doing this because I'm developing Firefox extension. I guess I have to use spies, but couldn't figure out how because my ajax won't return anything.

    submit : function() {
        var url = window.arguments[0];
        var request = new XMLHttpRequest();
        request.open("POST", 'http://'+this.host+'/doSomething', true);
        request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        request.send("param="+param+"&emotions="+this.getParams());
        request.onreadystatechange = function() {
            if(this.readyState == 4) {
                // alert(this.responseText);
            }
        };

    }