Add #deleted? method to PlaylistItem

[Riveascore]

#deleted? exists on Video.
But appears to only be accessible if you have proper auth on the Video object itself.

Say you have a Playlist, and you want to do something with each item.
You'll get a request error if the Video on a PlaylistItem is deleted.

But you can't call playlist_item.video.deleted?, you'll just get this error instead:

Yt::Errors::NoItems: A request to YouTube API returned no items but some were expected

[kangkyu]

Thank you for reporting @Riveascore