Bug 11912: (refactoring followup) make GetMarcISBN implement its advertised API

Bug 11912: (refactoring followup) make GetMarcISBN implement its advertised API

The current implementation of GetMarcISBN contradicts the documented API.
It currently returns an array of hashes with only one key (marcisbn)
which doesn't add any value to it.

I chose to fix GetMarcISBN to honour the API instead of changing thex
 docs, because it seems a really silly change.

To test:
- Run:
  prove t/db_dependent/Biblio.t
=> SUCCESS
- catalogue/detail.pl should correctly show ISBNs.
- opac/opac-detail.pl should correctly show ISBNs in both prog and bootstrap.
- opac-opac-sendshelf.pl should correctly show ISBNs in the email.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>