#39: make lastPlayed key optional
#39: make lastPlayed key optional

dvirtz/gmusicapi,simon-weber/Unofficial-Google-Music-API,thebigmunch/gmusicapi,TheOpenDevProject/gmusicapi,nvbn/Unofficial-Google-Music-API,tanhaiwang/gmusicapi,dvirtz/gmusicapi,peetahzee/Unofficial-Google-Music-API,simon-weber/gmusicapi,jimyx17/gmusic