The /contacts/:id edit form always submits the `private` checkbox, but
`current_value/2` had no `"private"` clause and fell through to the
catch-all `nil`. The Contact schema defaults `private: false`, so an
unchanged box compared `false != nil` and was always tagged as a
change — for non-owners that meant a spurious pending review queued
on every save, and for owners/admins it triggered a no-op direct
update.
Adds the missing clause so the diff sees boolean-vs-boolean and
correctly returns `:no_changes` when the box is untouched.
Graham McIntire
13996583bd