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_desktop_notifications/toast.cc
new_size.cy = margin_.cy + (tm_cap.tmHeight) + margin_.cy;
      if (!data_->body_text.empty())
        new_size.cy += margin_.cy + (3 * tm_body.tmHeight);
      if (data_->image) {
        BITMAP bm;
        if (GetObject(data_->image, sizeof(bm), &bm)) {
          
          const int max_dim_size = 80;
          auto width = bm.bmWidth;
          auto height = bm.bmHeight;
          if (width < height) {
            if (height > max_dim_size) {
              width = width * max_dim_size / height;
              height = max_dim_size;
            }
          } else {
            if (width > max_dim_size) {
              height = height * max_dim_size / width;
              width = max_dim_size;
            }
          }
          ScreenMetrics scr;
          SIZE image_draw_size = {scr.X(width), scr.Y(height)};
          new_size.cx += image_draw_size.cx + margin_.cx;
          auto height_with_image =
              margin_.cy + (image_draw_size.cy) + margin_.cy;
          if (new_size.cy < height_with_image)
            new_size.cy = height_with_image;
          UpdateScaledImage(image_draw_size);
        }