iterate: fix minimisation downgrade when encountering authoritative referrals

iterate: fix minimisation downgrade when encountering authoritative referrals

This fixes turning off minimisation when there's an authoritative referral
answer on the resolution path. This happens when there's a nameserver,
which is authoritative for both parent and child side of the delegation,
so it answers from the child side with AA=1. Such answer will be mistakenly
processed as authoritative, and QNAME minimisation will be turned off
(assuming this is the final zone cut).