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
case WM_LBUTTONDOWN: {
      auto* inst = Get(hwnd);
      inst->Dismiss();
      Notification notification(inst->data_);
      if (inst->is_close_hot_)
        inst->data_->controller->OnNotificationDismissed(notification);
      else
        inst->data_->controller->OnNotificationClicked(notification);
    }
      return 0;
    case WM_MOUSEMOVE: {
      auto* inst = Get(hwnd);
      if (!inst->is_highlighted_) {
        inst->is_highlighted_ = true;
        TRACKMOUSEEVENT tme = {sizeof(tme), TME_LEAVE, hwnd};
        TrackMouseEvent(&tme);
      }
      POINT cursor = {GET_X_LPARAM(lparam), GET_Y_LPARAM(lparam)};
      inst->is_close_hot_ =
          (PtInRect(&inst->close_button_rect_, cursor) != FALSE);
      if (!inst->is_non_interactive_)
        inst->CancelDismiss();
      inst->UpdateContents();
    }
      return 0;
    case WM_MOUSELEAVE: {
      auto* inst = Get(hwnd);
      inst->is_highlighted_ = false;
      inst->is_close_hot_ = false;
      inst->UpdateContents();