Bug 19369: Add helper function for pagination attributes generation

Bug 19369: Add helper function for pagination attributes generation

This patch introduces a new helper function to the Koha::REST::Plugin::Pagination
plugin, called 'dbic_merge_pagination'.

This simple function adds SQL::Abstract pagination attributes ('page' and 'rows') to the
passed $filter hashref.

To test:
- Apply this patches
- Run:
  $ koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/Koha/REST/Plugin/Pagination.t
=> SUCCESS: Tests pass!
- Sign off :-D

Sponsored-by: Camden County
Signed-off-by: Lari Taskula <lari.taskula@jns.fi>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>