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
TEXTMETRIC tm_cap;
  SelectFont(hdc_, caption_font);
  GetTextMetrics(hdc_, &tm_cap);
  auto text_offset_x = margin_.cx;
  BITMAP image_info = {};
  if (scaled_image_) {
    GetObject(scaled_image_, sizeof(image_info), &image_info);
    text_offset_x += margin_.cx + image_info.bmWidth;
  }
  
  POINT close_pos;
  {
    SIZE extent = {};
    GetTextExtentPoint32W(hdc_, &close, 1, &extent);
    close_button_rect_.right = toast_size_.cx;
    close_button_rect_.top = 0;
    close_pos.x = close_button_rect_.right - margin_.cy - extent.cx;
    close_pos.y = close_button_rect_.top + margin_.cy;
    close_button_rect_.left = close_pos.x - margin_.cy;
    close_button_rect_.bottom = close_pos.y + extent.cy + margin_.cy;
  }
  
  if (scaled_image_) {
    HDC hdc_image = CreateCompatibleDC(NULL);
    SelectBitmap(hdc_image, scaled_image_);
    BLENDFUNCTION blend = {AC_SRC_OVER, 0, 255, AC_SRC_ALPHA};
    AlphaBlend(hdc_, margin_.cx, margin_.cy, image_info.bmWidth,
               image_info.bmHeight, hdc_image, 0, 0, image_info.bmWidth,
               image_info.bmHeight, blend);
    DeleteDC(hdc_image);