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
if (GetDIBits(hdc_screen, bitmap, 0, 0, 0,
                    reinterpret_cast<BITMAPINFO*>(&bmi), DIB_RGB_COLORS) &&
          bmi.biSizeImage > 0 && (bmi.biSizeImage % 4) == 0) {
        auto* buf = reinterpret_cast<BYTE*>(
            _aligned_malloc(bmi.biSizeImage, sizeof(DWORD)));
        if (buf) {
          GetDIBits(hdc_screen, bitmap, 0, bm.bmHeight, buf,
                    reinterpret_cast<BITMAPINFO*>(&bmi), DIB_RGB_COLORS);
          const DWORD* src = reinterpret_cast<DWORD*>(buf);
          const DWORD* end = reinterpret_cast<DWORD*>(buf + bmi.biSizeImage);
          BYTE* dest = reinterpret_cast<BYTE*>(alpha_src_bits);
          for (; src != end; ++src, ++dest) {
            BYTE a = *src >> 24;
            *dest++ = a;
            *dest++ = a;
            *dest++ = a;
          }
          _aligned_free(buf);
        }
      }
    }
  }
  if (alpha_src_bitmap) {
    BITMAPINFOHEADER bmi = {sizeof(BITMAPINFOHEADER)};
    bmi.biWidth = width;
    bmi.biHeight = height;
    bmi.biPlanes = 1;
    bmi.biBitCount = 32;
    bmi.biCompression = BI_RGB;
    void* color_bits;