创建 XMLHttpRequest 对象

 var _createHttpRequest = function () {
     var obj;
     if (window.XMLHttpRequest) obj = new XMLHttpRequest();
     else if (window.ActiveXObject) {
         try {
             obj = new ActiveXObject("Msxml2.XMLHTTP");
         }
         catch (e0) {
             try {
                 obj = new ActiveXObject("Microsoft.XMLHTTP");
             }
             catch (e1) { }
         }
     }
     return obj;
 }