当同时触发超过4个异步XmlHttpRequests时，IE8挂起

for (var i = 0; i < 5; ++i) {
  var xhr;
  if (window.XMLHttpRequest) {
    xhr = new XMLHttpRequest();
  } else if (window.ActiveXObject) {
    xhr = new ActiveXObject("Msxml2.XMLHTTP");
  }
  xhr.open('GET', '/Test/LongOperation?p=' + new Date());
  xhr.send('');
}
for (var i = 0; i < 5; ++i) {
  var xhr;
  if (