Type.extra (#1538)

* Revert "pkg/typify: empty messages are OtherError (#1531)"

This reverts commit fc1d73ffa9.

* plugin/cache: add failsafeTTL

If we can not see what TTL we should put on a message to be cached, use
5 seconds as minimal TTL. We used to apply the maximum TTL to these
  messages.

plugin/cache/cache_test.go

@@ -22,7 +22,6 @@ type cacheTestCase struct {
 	Authoritative      bool
 	RecursionAvailable bool
 	Truncated          bool
-	Response           bool
 	shouldCache        bool
 }
 
@@ -112,15 +111,6 @@ var cacheTestCases = []cacheTestCase{
 		},
 		shouldCache: false,
 	},
-	{
-		// Response with only something in the additional, this should not be cached.
-		Response: true,
-		in: test.Case{
-			Qname: "example.org.", Qtype: dns.TypeMX,
-			Extra: []dns.RR{test.MX("example.org.	1800	IN	MX	1 mx.example.org.")},
-		},
-		shouldCache: false,
-	},
 	{
 		RecursionAvailable: true, Authoritative: true,
 		Case: test.Case{
@@ -150,7 +140,6 @@ func cacheMsg(m *dns.Msg, tc cacheTestCase) *dns.Msg {
 	m.AuthenticatedData = tc.AuthenticatedData
 	m.Authoritative = tc.Authoritative
 	m.Rcode = tc.Rcode
-	m.Response = tc.Response
 	m.Truncated = tc.Truncated
 	m.Answer = tc.in.Answer
 	m.Ns = tc.in.Ns