Headers were built and rebuilt as "Mime-Version" rather than
"MIME-Version". Both are valid per RFC 2045, but some spam filters,
such as rspamd, score the mixed-case form as a deliverability signal.
Fix the header name in the accessor macro and in ParsedHeader's
grammar table, the places that spell it, and update the snapshot
tests across mailparsing, message, kumo-log-types, mod-mimepart, and
kumod that captured the old spelling, along with the reference docs
that showed the old casing in their examples. #564
Wez Furlong