XWIKI-9013: Multiple Select List values are not correctly indexed by Lucene
* Add AbstractNotifyOnUpdateList#toString() for backwards compatibility and a few unit tests that were missing..
(cherry picked from commit 578d992bbcbb24c3b80664605659c68bd0bfa9ef)

Conflicts:
	xwiki-platform-core/xwiki-platform-oldcore/pom.xml