After creating a notification and passing a tag, then subsequent notifications with the same tag don't show up for the entire lifespan of the app.

* Electron version: 1.8.1
* Operating system: Windows 7 x64

### Expected behavior

New notifications should open after the last one has closed.

### Actual behavior

Nothing happens

### How to reproduce

  $ git clone https://github.com/electron/electron-quick-start .
  $ npm install
  $ npm start

Then in the app open dev tools and paste this in the console:

    new Notification("test", { tag: "test" });

Wait for the notification to disappear and paste it again, observe nothingness.
atom/browser/notifications/win/win32_notification.cc
bmi.biPlanes = 1;
      bmi.biBitCount = 32;
      bmi.biCompression = BI_RGB;
      HDC hdcScreen = GetDC(NULL);
      image =
          CreateDIBitmap(hdcScreen, &bmi, CBM_INIT, options.icon.getPixels(),
                         reinterpret_cast<BITMAPINFO*>(&bmi), DIB_RGB_COLORS);
      ReleaseDC(NULL, hdcScreen);
    }
  }
  Win32Notification* existing = nullptr;
  if (!options.tag.empty())
    existing = presenter->GetNotificationObjectByTag(options.tag);
  if (existing) {
    existing->tag_.clear();
    this->notification_ref_ = std::move(existing->notification_ref_);
    this->notification_ref_.Set(options.title, options.msg, image);
  } else {
    this->notification_ref_ =
        presenter->AddNotification(options.title, options.msg, image);
  }
  this->tag_ = options.tag;
  if (image)
    DeleteObject(image);
}
void Win32Notification::Dismiss() {
  notification_ref_.Close();
}
}