middleware/file: Support delegations (#124)

Return a delegation when seeing one while traversing the tree in
search of an answer.

Put the SOA and NS record in the zone.Apex as these are to be handled
somewhat special.

Lowercase record on insert to make compares easier. This lowercases
all RR that have domain names in their rdata as well.

middleware/file/file.go

@@ -80,18 +80,15 @@ func (f File) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 	m := new(dns.Msg)
 	m.SetReply(r)
 	m.Authoritative, m.RecursionAvailable, m.Compress = true, true, true
+	m.Answer, m.Ns, m.Extra = answer, ns, extra
 
 	switch result {
 	case Success:
-		m.Answer = answer
-		m.Ns = ns
-		m.Extra = extra
-	case NameError:
-		m.Ns = ns
-		m.Rcode = dns.RcodeNameError
-		fallthrough
 	case NoData:
-		m.Ns = ns
+	case NameError:
+		m.Rcode = dns.RcodeNameError
+	case Delegation:
+		m.Authoritative = false
 	case ServerFailure:
 		return dns.RcodeServerFailure, nil
 	}