Previously we would seek to the beginning of the io.ReadSeeker. It is much more
flexible to assume that the caller passes the ReadSeeker in an appropriate state.
The Seek in SumID3v2() didn't take into account the size of the ID3v2 Header
(10 bytes). Thus when tagging the same file with or without it's tag, there was
a discrepancy. (
And I've added a default case when parsing the id3v2 tags, by storing the
binary content to Raw()
