Fix case of MIME-Version header field name

[A1bi]

This changes the header field name Mime-Version to MIME-Version.

While an incorrect case here might not cause any trouble since header field names should be treated case-insensitively, the actual correct spelling of this header field is MIME-Version (see RFC 2045).

More importantly I noticed that spam filters such as Rspamd penalize emails with an incorrect case for header fields like this, see the screenshot below.