playlist: fix mpd_playlist_dup so it also copies "last_modified" attribute
playlist: fix mpd_playlist_dup so it also copies "last_modified" attribute

jwise/libmpdclient,jwise/libmpdclient,sl0th/libmpdclient,jwise/libmpdclient,yusiwen/libmpdclient,yusiwen/libmpdclient,yusiwen/libmpdclient,sl0th/libmpdclient,sl0th/libmpdclient