Bug 19537: (QA follow-up) Consistent structures

Bug 19537: (QA follow-up) Consistent structures

The subfield_loop structure should contain things like:
$VAR1 = {
          'marc_subfield' => 'v',
          'marc_values' => [
                             'form'
                           ]
        };

This patches replaces the list by an array ref to restore consistency,
although Template Toolkit does not really care. Both work.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>