PlaylistSave does not save until max_size is hit

[myselfhimself]

Hello,

looking at PlaylistSave.next_track(), the buffer is saved into Spotify only when the buffer size gets bigger than the the constructor's max_size limit. However, when the pipeline's source yields no more tracks, the less-than-max_size buffer is not flushed into Spotify.

I will put a pull request for that.