Fix issue with date format yyyy-mm

MB sometimes gives dates in form yyyy-mm. Mutagen cannot save id3v23
dates in this format and truncates them to yyyy. Then they don't match
when the file is reloaded.

This fix detects New metadata dates in this format and truncates them.
Fix stupid coding error in mimeo comment response.
Merge branch 'sophist_fix_id3v23_date_yyyy-mm' of https://github.com/Sophist-UK/picard into mb-master
Merge branch 'master' of github.com:musicbrainz/picard into mb-master