Set new ORIGINALDATE tag for m4a files in addition to existing "ORIGINAL

YEAR" tag. The former is used by Plex Media Server to discern the
original release date. Fixes #70

This behavior also matches other popular tagging libraries, such as https://github.com/Borewit/music-metadata

mediafile.py

@@ -2087,6 +2087,7 @@ def as_dict(self):
     original_date = DateField(
         MP3StorageStyle('TDOR'),
         MP4StorageStyle('----:com.apple.iTunes:ORIGINAL YEAR'),
+        MP4StorageStyle('----:com.apple.iTunes:ORIGINALDATE'),
         StorageStyle('ORIGINALDATE'),
         ASFStorageStyle('WM/OriginalReleaseYear'))