Do not reset draft info for drafts

* src/cryptcontroller.cpp (CryptController::update_mail_mapi):
Pass isDraftEncrypt to finalize message.
* src/mimemaker.cpp (finalize_message): Do not remove draft info
for drafts.

This commit is a bit confusing because we use "draft info" as
our tracking property for the encrypt/sign actions. Now that
we also work with real drafts this is a bit confusing.