The mail-gem tries to reorder all mime-parts so that e.g. text/plain
comes first. An unreleased patch in upstream
<https://github.com/mikel/mail/commit/7812a77>  allows to avoid that
reordering, but even then we'd have to monkey-patch because mail-gpg
replaces the body-instances and thus the manually specified sorting
order is lost.

Closes #181.