fix(task): eventTask/Periodical task should not be reset after being cancelled when running

[JiaLiPassion]

the issue can be described as the following cases.

     const zone = Zone.current.fork({name: 'testZone'});

     const task = zone.scheduleMacroTask('testPeriodTask', () => {
       zone.cancelTask(task);
     }, {isPeriodic: true}, () => {}, () => {});

    task.invoke();
    expect(task.state).toBe('notScheduled');

    
     const task = zone.scheduleEventTask('testEventTask', () => {
       zone.cancelTask(task);
     }, null, () => {}, () => {});

    task.invoke();
    expect(task.state).toBe('notScheduled');

When task is cancelled in task's callback(running state), the runTask logic here https://github.com/angular/zone.js/blob/master/lib/zone.ts#L757 will reset the task's state to scheduled, so it will cause issues such as #638, updateTaskCount cause null error, because the task is cancelled but still in scheduled state.

[naveedahmed1]

Can this be released anytime soon? Because it breaks Angular applications in most of the cases.

[naveedahmed1]

Thank you so much @mhevery my Angular 2 app is stable again after this update :)