Bug 33328: Rename x-marc-schema => x-record-schema

Bug 33328: Rename x-marc-schema => x-record-schema

This patch renames the header on the biblio API so it is not entirely
tied to MARC, trying to follow the biblio_metadata table generic
approach.

To test:
1. Apply this patch
2. Run:
   $ prove t/db_dependent/api/v1/biblios.t
=> SUCCESS: Tests pass
3. You agree with the change?
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Magnus Enger <magnus@libriotech.no>
Change sounds reasonable. Tests pass before and after patch.
"grep -r "x-marc-schema" *" confirms no occurrences after patch.
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>