Bug 32336: Fix encoding of MARCXML output for REST API (UNIMARC)

Bug 32336: Fix encoding of MARCXML output for REST API (UNIMARC)

Test plan:
1. Be sure to test on a UNIMARC instance
2. Enable system preferences RESTPublicAPI and RESTPublicAnonymousRequests
3. Create a biblio with some diacritics
4. Restart koha and do not interact with OPAC or the staff interface
   (this may change $MARC::File::XML::RecordFormat)
5. Query the public API to retrieve the created biblio in MARCXML. For
   instance with cURL:

   curl -H 'Accept: application/marcxml+xml' \
    http://koha.local/api/v1/public/biblios/<biblionumber>

   You should see encoding issues.
6. Apply the patch and restart Koha.
7. Repeat step 5.
   You should see no encoding issues.
8. You should also test /api/v1/biblios/<biblionumber> (this one
   requires authentication, so cURL might not be the most practical
   tool; use your favorite tool for this)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>