Bug 26601: Add utf8 encoding to text output of overdue_notices.pl

Bug 26601: Add utf8 encoding to text output of overdue_notices.pl

This patch sets the UTF8 encoding for output of overdue_notices.pl when
outputting to txt.

To test:
0/ Check out an item with due date yesterday (use specify due date)
1/ Create ODUE text with utf8 encoded characters, chinese characters, umlauts etc
   <url>/cgi-bin/koha/tools/letter.pl?op=add_form&module=circulation&code=ODUE
2/ Run
   perl overdue_notices.pl -text ./
3/ Verify encoding in the generated text file is incorrect
4/ Apply patch and repeat 2/
3/ Confirm text now inludes properly encoded characters

Sponsored-by: Styrian State Library
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
With German umlauts on my system the editor still showed the
characters correctly (probably guessing the right encoding).
Using Chinese made the problem visible.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>