Bug 13757: (followup) Only touch opac_editable attributes

Bug 13757: (followup) Only touch opac_editable attributes

As reported by Owen, the members-update.pl was showing every attributes
the patron has (display issue) instead of showing only those affected by
the changes.

This patch fixes this by filtering the patron's attributes by opac
editability.

It also fixes Koha::Patron::Modification->approve so it only clears the
attributes with the updating 'code' and leaves the others untouched.
As its been coded so far (until someone refactors it all) the
Koha::Patron::Modification object needs to contain all the attributes
for a specific code. And it comes from parsing the UI's input.

Tests for Koha::Patron::Modification->approve to come.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
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>