message: Add tests for append_device_id function.
diff: @@ -2190,4 +2190,22 @@ mod tests {
  assert_eq!(track_id1, uri1);
  assert_eq!("spotify:track:1301WleyT98MSxVHPZCA6M", &uri2);
  }
  +
  + #[test]
  + fn test_append_device_id_without_question_mark() {
  + let path = "me/player/play";
  + let device_id = Some("fdafdsadfa".to_owned());
  + let spotify = SpotifyBuilder::default().build().unwrap();
  + let new_path = spotify.append_device_id(path, device_id);
  + assert_eq!(new_path, "me/player/play?device_id=fdafdsadfa");
  + }
  +
  + #[test]
  + fn test_append_device_id_with_question_mark() {
  + let path = "me/player/shuffle?state=true";
  + let device_id = Some("fdafdsadfa".to_owned());
  + let spotify = SpotifyBuilder::default().build().unwrap();
  + let new_path = spotify.append_device_id(path, device_id);
  + assert_eq!(new_path, "me/player/shuffle?state=true&device_id=fdafdsadfa");
  + }
  }
  