Failed to download Mix playlists

The `fetch_playlist_infos` function returns an error message ``` JSON error while Failed to parse playlist metadata: missing field `uploader` at line 1 column 629400 ``` for a Mix playlist link, e.g. https://www.youtube.com/watch?v=-Qwf-ps1dx0&list=RD-Qwf-ps1dx0&start_radio=1