Some MTAs try to normalize the case of all headers (including signed
text parts headers). Unfortunately, Mime-Version can be normalized to
different casing depending on the implementation (MIME- vs Mime-).

Since the signature is computed on the whole part, including its header,
changing the case can cause the signature to become invalid.

Due to how multipart/signed messages are constructed, we need to hack
around go-message writers to intercept the writing of a text part,
compute its signature and write the actual message with the proper
headers.

Unfortunately, go-message does not allow creating a message writer that
does not insert a Mime-Version header. This causes the Mime-Version
header to be inserted in the wrong place: it is put inside the signed
text part header instead on the top level header. Thus, included in the
signed content.

Make sure to remove any Mime-Version header from the signed part header.
Finally, ensure that Mime-Version is set on the top-level header so that
messages are compliant with RFC 2045.

Fixes: https://todo.sr.ht/~rjarry/aerc/143
Link: emersion/go-message#165
Link: emersion/go-pgpmail#15
Link: https://lists.sr.ht/~rjarry/aerc-devel/%3CCQRPF5EA0TF8.PEJ4AKCEGMFM%40fembook%3E
Changelog-fixed: `Mime-Version` is no longer inserted in signed
 text parts headers. MTAs normalizing header case will not corrupt
 signatures anymore.
Reported-by: Coco Liliace <chloe@liliace.dev>
Reported-by: Kirill Chibisov <contact@kchibisov.com>
Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Tim Culverhouse <tim@timculverhouse.com>