在 beforeSend中设置ajax请求的Content-type

$.ajaxSetup({
        beforeSend: function (xhr, settings) {
            if (settings.type == "POST") {
                if (!settings.data) {
                   var data = {};
                    data.token = "1";
                    settings.data = $.param(data);
                    xhr.setRequestHeader("Content-type", settings.contentType);// 设置Content-type
                }
            }
        }
    });