Use private policy instead of protected for attributes_with_values

Change-Id: I7ae59f5f9f307eb8d2fea9753c420e236d2a1502
Reviewed-on: http://review.couchbase.org/26819
Reviewed-by: Andrey Koleshko <ka8725@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>