Bug 19278: Make the pagination plugin use RESTdefaultPageSize

This bug introduces a new syspref RESTdefaultPageSize that is used on
the Koha::REST::Plugin::Pagination plugin to default to a valid value
for the page size if the param is not present on the query.

This patch also considers the situation in which no 'page' param is
passed, and no header should be added.

To test:
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/Koha/REST/Plugin/Pagination.t
=> FAIL: Tests fail because the plugin doesn't behave as the tests
expect.
- Apply this patch
- Run:
 k$ prove t/Koha/REST/Plugin/Pagination.t
=> SUCCESS: Tests pass!
- Sign off :-D

Sponsored-by: Camden County
Sponsored-by: ByWater Solutions

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>

Koha/REST/Plugin/Pagination.pm

@@ -60,7 +60,11 @@ It also adds X-Total-Count, containing the total results count.
 
             my $total    = $args->{total};
             my $req_page = $args->{params}->{_page};
-            my $per_page = $args->{params}->{_per_page};
+            my $per_page = $args->{params}->{_per_page} //
+                            C4::Context->preference('RESTdefaultPageSize');
+
+            # do we need to paginate?
+            return $c unless $req_page;
 
             my $pages = int $total / $per_page;
             $pages++