XMLHTTPRequest abort multipletimes will cause updateTaskCount incorrect

[JiaLiPassion]

Based on #287, and the comment from @RicardoVaranda's comment, the XMLHTTPRequest will cause Uncaught Error: More tasks executed then were scheduled. The error can be described
as

const req = new XMLHttpRequest();
      req.open('get', '/', true);
      req.send();
      req.addEventListener('readystatechange', function(ev) {
        if (req.readyState >= 2) {
          req.abort();
        }
      });

The abort will be called multiple times before the request is done.
So we should add logic to verify whether the request has already been aborted.