Fix collection detaching at the wrong time in attribute()

Summary:
Found in FatCRM where changes to collection attributes were not stored
anymore.

Test Plan:
New unittest to ensure that we get the attribute from the
detached collection, not from the original one.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19741