bug 10549: make the ILS-DI services advertise that they return UTF-8

bug 10549: make the ILS-DI services advertise that they return UTF-8

This patch changes a few occurences of ISO-8859-1 to UTF-8
within the XML generation of the ILS-DI module.

To test:
- Activate ILS-DI system preference
- Go to [youropac]/cgi-bin/koha/ilsdi.pl
- Check all examples in the documentation for the correct
  encoding
- Check GetAvailability gives you the correct encoding and
  check the source for the correct encoding

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 2a1d0fc6d2d2674d4182ed1c6be557b568778834)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>