Bug 21875: Handling subject line in Letters.pm

Bug 21875: Handling subject line in Letters.pm

The variable name $utf8 is very misleading: it contains MIME-Header encoding.
$message->{subject} comes from the database and is in perl internal format;
it should NOT be decoded as a MIME-Header.
After encoding to MIME-Header, previously another (useless) encoding to
UTF-8 was done. Since the string is plain ASCII, this is useless and
theoretically wrong. We should stay in MIME-Header.

Test plan:
See Bugzilla comment5.

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: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>