Bug 29330: (QA follow-up) Change to message/rfc822

Bug 29330: (QA follow-up) Change to message/rfc822

This content-type might be more appropriated to use as a temporary
label for the serialized email message with attachments.

WARNING: perl -cw tells you that the constant is redefined. This has
to do with an already existing module dependency loop of Letters.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>