Be deterministic when saving to file #329
I am using Unison as a backup tool for my music files and every time I update my library's tags with Mutagen some of them are found modified although the audio part and the tags had not been modified. I found out that APEV2 tags are sorted by lengths before saving to file but this is not sufficient. Tags that have the exact same length are saved in random orders because of Python's dictionary implementation. This patch aims to correct this because it adds tags keys to the comparing function and since tag's keys are unique, this ensures that each sorting key is unique too.