Delegations coming from mirror zones are missing DS RRsets and glue

When a delegation coming from a mirror zone is sent in response to a non-recursive query, DS RRsets and/or glue are missing. This is not a big issue since mirror zones only really need to work with recursive queries, but fixing this would allow the split-view configuration from RFC 7706 section B.1 to still work if the user inserts mirror yes; into it without fully understanding what mirror zones are.