Bug 27680: Allow traditional multi-params to work

Bug 27680: Allow traditional multi-params to work

This patch allows the preceeding test to pass. Strictly, we want a comma
delimited string for our _order_by parameter, but we cannot easily block
a traditional multi-passed parameter.  As such the 'nice' thing to do is
handle it when such a thing is passed as it will pass through validation
regardless.

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>