Genre tag doesn't support setting with VERSION_BOTH

[trydalch]

Even though the tag is in both V1 and V2 tags, setting genre is not supported with VERSION_BOTH is set.

The other tags either support this, or don't throw an exception when attempted.

[hejops]

Possibly related; writing genre to a file creates corrupt v1 tags. This file did not previously contain v1 tags:

    mp3 = m.MP3File(f)
    mp3.genre = '...'
    mp3.set_version(m.VERSION_2)
    mp3.save()

Tag 1: ID3v1.1
  Title         ªªªªªªªªªªªªªªªªªªªªªÿûdÝð
  Artist        i
  Album         ªªªªªªªªªªªªªªªªªªªªªªªªªªªªªª
  Comment       ªªªªªªªªªªªªªªªªªªªªªªªªªªªª
  Track Number  170
  Genre         Leftfield