EasyID3.RegisterTextKey(...,'COMM') fails on retrieve

[Simon Lambourn]

EasyID3.RegisterTextKey('comment', 'COMM') actually defines a frame called 'COMM::XXX', which can't be retrieved by the same name.

You can set this field as follows

>>> e = EasyID3()
>>> e['comment'] = 'a comment'
>>> vars(e)
{'_EasyID3__id3': {'COMM::XXX': COMM(encoding=<Encoding.UTF8: 3>, lang='XXX', desc='', text=['a comment'])}}

but attempting to retrieve it causes a KeyErrror:'COMM'

Code:
>>> e['comment']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/mutagen/easyid3.py", line 213, in __getitem__
    return func(self.__id3, key)
  File "/usr/lib/python3/dist-packages/mutagen/easyid3.py", line 121, in getter
    return list(id3[frameid])
  File "/usr/lib/python3/dist-packages/mutagen/_util.py", line 536, in __getitem__
    return self.__dict[key]
KeyError: 'COMM'

As a workaround, if I define a second text key
>>> EasyID3.RegisterTextKey('comment2', 'COMM::XXX')
I can now retrieve the original tag

>>> vars(e)
{'_EasyID3__id3': {'COMM::XXX': COMM(encoding=<Encoding.UTF8: 3>, lang='XXX', desc='', text=['a comment'])}}
>>> e['comment2']
['a comment']

Expected behaviour: e['comment'] returns 'a comment'

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: python3-mutagen 1.40.0-2
ProcVersionSignature: Ubuntu 5.0.0-20.21-generic 5.0.8
Uname: Linux 5.0.0-20-generic x86_64
ApportVersion: 2.20.10-0ubuntu27
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Jul 4 15:13:27 2019
Dependencies:

PackageArchitecture: all
SourcePackage: mutagen
UpgradeStatus: Upgraded to disco on 2019-04-25 (70 days ago)