Bug 13757: Make K::P::Modifications->pending return K::P::Attribute

Bug 13757: Make K::P::Modifications->pending return K::P::Attribute

This patch makes Koha::Patron::Modifications->pending return
Koha::Patron:Attribute objects. They are not stored on the DB but only
live in memory on runtime.

members-update.pl is the only place this is used, and this way we have
all we need for better rendering the UI.

Tests are added for the changed API.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>