Bug 27680: Add support for param[] syntax

Bug 27680: Add support for param[] syntax

While not that common nowadays, it is the syntax PHP uses, and
DataTables also generates such thing in 'traditional' mode. We should
support it as well.

This patch adds support for that. It does so by adding _order_by[] to
the reserved param names, and proper handling on the dbic_merge_sorting
helper.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/REST/Plugin/Objects.t
=> SUCCESS: Tests pass!
3- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Bug 27680: (QA follow-up) Minor perlcritic issue

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>