Bug 33329: Correctly handle UNIMARC encoding for biblios

Bug 33329: Correctly handle UNIMARC encoding for biblios

This patch introduces the same treatment we had for bug 32336, this time
for Koha::Objects::Record::Collections.

To test:
1. Apply the regression tests patch
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/api/v1/biblios.t
=> FAIL: Tests fail! Encoding is wrong for UNIMARC!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>