Fixed modified_offset bugs, set_version() bug.
Fixed modified_offset bugs, set_version() bug.

- Fixed a bug where modified_offset would be set to 0 when new frames
  would be removed
- Fixed modified_offset not taking the min value when removing a picture
  by type
- Perform no action in ID3Tag::set_version() when the version doesn't
  change

sp3d/rust-id3