fix bug add event listener to xhrhttprequest multiple times

[JiaLiPassion]

in issue #287, the xhrhttprequest add documentreadychange eventlistener multipletimes (https://github.com/angular/zone.js/blob/master/lib/browser/browser.ts#L66), so when task finished, updateTaskCount will cause "More tasks executed then were scheduled" error. The issue
can be simplified as the following test case.

 Zone.current.fork({name:'test'}).run(function() {
      var req = new XMLHttpRequest();
      req.open('get', '/', true);
      req.send();
      req.onloadend = function() {
        req.open('get', '/', true);
        req.send();
      }

So we should removeEventlistener before add new one.