Bug 16578: Fix wide character errors in opac-export.pl

Bug 16578: Fix wide character errors in opac-export.pl

This simple patch fixes wide character warning raised by opac-export.pl when
'MARC (Unicode/UTF-8)' and 'MARC (Unicode/UTF-8, Standard)' formats are chosen
for downloading records.

To test:
- Have records indexed
- Open your OPAC logs:
  $ tail -f /var/log/koha/kohadev/opac-error.log
- Open the detail page in the opac for a record
- Choose Save record > MARC (Unicode/UTF-8)
=> FAIL: opac-export.pl: Wide character in print at /home/vagrant/kohaclone/opac/opac-export.pl line 116., referer: http://localh...
- Choose Save record > MARC (Unicode/UTF-8, Standard)
=> FAILE opac-export.pl: Wide character in print...
- Apply the patch
- Choose Save record > MARC (Unicode/UTF-8)
=> SUCCESS: No warnings raised.
- Choose Save record > MARC (Unicode/UTF-8, Standard)
=> SUCCESS: No warnings raised.
- Sign off :-D

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 7d4844e171b1319b90e7bd73510bd97c2acbf635)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>