Create the script tag given the specified url, params and options. The options
parameter is passed to allow an override to access it.
@private
@param {String} url The url of the request
@param {Object} params Any extra params to be sent
@param {Object} options The object passed to {@link #request}.
function(url, params, options) {
        var script = document.createElement('script');
        script.setAttribute("src", Ext.urlAppend(url, Ext.Object.toQueryString(params)));
        script.setAttribute("async", true);
        script.setAttribute("type", "text/javascript");
        return script;
    }