Assume ISO-8859-1 when charset is unknown or bogus

Unfortunately, this is necessary because some mail clients send in content without defining
the charset or at worse using freaking wrong/invalid charsets like
'default'!
Merge pull request #551 from protich/issue/encoding

Assume ISO-8859-1 when charset is unknown or bogus

Reviewed-By: Jared Hancock <jared@osticket.com>