Bug 10269: Use 'reply-to' address if passed to EnqueueLetter

Bug 10269: Use 'reply-to' address if passed to EnqueueLetter

This patch updates the EnqueueLetter method to allow passing a reply_to
address. This allows for passing in arbitrary addresses for the reply
address which will not override the from address. This improves the
chances of your email getting past spam filters.

Signed-off-by: Myka Kennedy Stephens <mkstephens@lancasterseminary.edu>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>