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
bool success = false;
  if (IsAppThemed()) {
    HKEY hkey;
    if (RegOpenKeyEx(HKEY_CURRENT_USER,
                     TEXT("SOFTWARE\\Microsoft\\Windows\\DWM"), 0,
                     KEY_QUERY_VALUE, &hkey) == ERROR_SUCCESS) {
      COLORREF color;
      DWORD type, size;
      if (RegQueryValueEx(hkey, TEXT("AccentColor"), nullptr, &type,
                          reinterpret_cast<BYTE*>(&color),
                          &(size = sizeof(color))) == ERROR_SUCCESS &&
          type == REG_DWORD) {
        
        color = RGB(GetRValue(color), GetGValue(color), GetBValue(color));
        success = true;
      } else if (RegQueryValueEx(hkey, TEXT("ColorizationColor"), nullptr,
                                 &type, reinterpret_cast<BYTE*>(&color),
                                 &(size = sizeof(color))) == ERROR_SUCCESS &&
                 type == REG_DWORD) {
        
        color = RGB(GetBValue(color), GetGValue(color), GetRValue(color));
        success = true;
      }
      RegCloseKey(hkey);