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
auto* color_bitmap =
        CreateDIBSection(NULL, reinterpret_cast<BITMAPINFO*>(&bmi),
                         DIB_RGB_COLORS, &color_bits, NULL, 0);
    void* alpha_bits;
    auto* alpha_bitmap =
        CreateDIBSection(NULL, reinterpret_cast<BITMAPINFO*>(&bmi),
                         DIB_RGB_COLORS, &alpha_bits, NULL, 0);
    HDC hdc = CreateCompatibleDC(NULL);
    HDC hdc_src = CreateCompatibleDC(NULL);
    if (color_bitmap && alpha_bitmap && hdc && hdc_src) {
      SetStretchBltMode(hdc, HALFTONE);
      
      SelectObject(hdc, color_bitmap);
      SelectObject(hdc_src, bitmap);
      StretchBlt(hdc, 0, 0, width, height, hdc_src, 0, 0, bm.bmWidth,
                 bm.bmHeight, SRCCOPY);
      
      SelectObject(hdc, alpha_bitmap);
      SelectObject(hdc_src, alpha_src_bitmap);
      StretchBlt(hdc, 0, 0, width, height, hdc_src, 0, 0, bm.bmWidth,
                 bm.bmHeight, SRCCOPY);
      
      GdiFlush();
      
      auto* dest = reinterpret_cast<BYTE*>(color_bits);
      auto* src = reinterpret_cast<const BYTE*>(alpha_bits);
      auto* end = src + (width * height * 4);
      while (src != end) {
        dest[3] = src[0];
        dest += 4;