Bug 24531: Fix OAI-PMH sets for records with repeated fields

Bug 24531: Fix OAI-PMH sets for records with repeated fields

Currently, an OAI-PMH set mapping will only match if the value it
is looking for occurs in the first instance of a repeated field.

To test:
- Apply the first patch with two new tests
- Run something like this:
  sudo koha-shell -c "prove -v t/db_dependent/OAI/Sets.t" kohadev
- Verify that the last test fails
- Apply this secind patch
- Rerun the test file above
- Verify that all tests now pass

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>