message: as_deref is clearer than as_ref
diff: @@ -1711,7 +1711,7 @@ impl Spotify {
  });
  let params = build_map! {
  optional "country": country.map(|x| x.as_ref()),
  - optional "additional_types": additional_types.as_ref(),
  + optional "additional_types": additional_types.as_deref(),
  };
  let result = self.endpoint_get("me/player", &params).await?;
  