fix(xhr): fix #657, sometimes xhr will fire onreadystatechange with done twice

[JiaLiPassion]

Fix #657.

The similar issue can be found here,https://bugs.chromium.org/p/chromium/issues/detail?id=159827

And some times the XMLHttpRequest will fire onreadystatechange with readyState=4 multiple times.

xhr.onreadystatechange = function() {
                    if (xhr.readyState === 4) {
                        if (xhr['ok']) {
                            console.log('error duplicate done');
                        }
                        xhr['ok'] = true;
                    }
                }

this will cause ZoneTask to try to transit from notScheduled to running and cause error.
So I add a check to make sure ZoneTask is in scheduled state before running.