message: fix optional
diff: @@ -721,7 +721,7 @@ impl Spotify {
  let uris = track_ids.into_iter().map(|id| id.uri()).collect::<Vec<_>>();
  let params = build_json! {
  "uris": uris,
  - "position": position,
  + optional "position": position,
  };
  let url = format!("playlists/{}/tracks", playlist_id.id());
  