Bug 19370: Add helper function for order_by attribute generation

Bug 19370: Add helper function for order_by attribute generation

This patch introduces a helper function called 'dbic_merge_sorting'
to Koha::REST::Plugin::Query.

This simple function adds SQL::Abstract order_by attribute to the passed
$filter hashref, as explained in the POD.

It introduces a syntax for passing sorting params on the request to the REST api.
The proposed syntax has been found in the wild, and is pretty trivial to parse/work with:

    GET /api/v1/<endpoint>?order_by=+column_1|-column_2|column_3

As explained on the POD, + stands for 'asc' and - for 'desc'. If ommited, it defaults to the
DB engine default (usually asc).

To test:
- Apply this patches
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass! And they make sense! :-P
- Sign off :-D

Edit: renamed params to match DBIC terminology. My bad :-D (tcohen)

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>