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
{
    
    
    
    constexpr float alpha = 0.9f;
    constexpr float intensity_light[] = {(1.0f * alpha), (0.8f * alpha)};
    constexpr float intensity_dark[] = {(0.1f * alpha), (0.3f * alpha)};
    
    auto& i = (back_luma < 0.6f) ? intensity_light : intensity_dark;
    float r, g, b;
    r = i[0] + back_f.r * (1 - alpha);
    g = i[0] + back_f.g * (1 - alpha);
    b = i[0] + back_f.b * (1 - alpha);
    fore_color = RGB(r * 0xff, g * 0xff, b * 0xff);
    r = i[1] + back_f.r * (1 - alpha);
    g = i[1] + back_f.g * (1 - alpha);
    b = i[1] + back_f.b * (1 - alpha);
    dimmed_color = RGB(r * 0xff, g * 0xff, b * 0xff);
  }
  
  {
    auto* brush = CreateSolidBrush(back_color);
    RECT rc = {0, 0, toast_size_.cx, toast_size_.cy};
    FillRect(hdc_, &rc, brush);
    DeleteBrush(brush);
  }
  SetBkMode(hdc_, TRANSPARENT);
  const auto close = L'\x2715';
  auto* caption_font = data_->controller->GetCaptionFont();
  auto* body_font = data_->controller->GetBodyFont();