Folder.savemessage() is supposed to return the new UID that a backend
assigned, and it BaseFolder.copymessageto() fails if we don't return a
non-negative number in the savemessage() there.
For some reason, the UIDMappedFolder was not returning anything in
savemessage, despite clearly stating in the code docs that it is
supposed to return a UID. Not sure how long this has already been the
case. This patch fixes the UIDMappedFolder to behave as it should.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>