Populate $oldSlug from $changeSet

Populating $oldSlug from $changeSet original value rather than from
$object. $object holds the new value at this stage. This meant that
other preUpdate event handlers could not see the original value.
Merge remote-tracking branch 'levo/master'

* levo/master:
  Populate $oldSlug from $changeSet
various fixes 2.3.6