Fix max-age in http server (#1890)

* Fix max-age in http server

Move the minMsgTTL to dnsutil and rename it MinimalTTL, move some
constants there as well.
Use these new function in server_https to correctly set the max-age
HTTP header.

Fixes: #1823

* Linter

plugin/pkg/dnsutil/ttl.go

@@ -0,0 +1,72 @@
+package dnsutil
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/response"
+
+	"github.com/miekg/dns"
+)
+
+// MinimalTTL scans the message returns the lowest TTL found taking into the response.Type of the message.
+func MinimalTTL(m *dns.Msg, mt response.Type) time.Duration {
+	if mt != response.NoError && mt != response.NameError && mt != response.NoData {
+		return MinimalDefaultTTL
+	}
+
+	// No data to examine, return a short ttl as a fail safe.
+	if len(m.Answer)+len(m.Ns)+len(m.Extra) == 0 {
+		return MinimalDefaultTTL
+	}
+
+	minTTL := MaximumDefaulTTL
+	for _, r := range m.Answer {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	for _, r := range m.Ns {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+
+	for _, r := range m.Extra {
+		if r.Header().Rrtype == dns.TypeOPT {
+			// OPT records use TTL field for extended rcode and flags
+			continue
+		}
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	return minTTL
+}
+
+const (
+	// MinimalDefaultTTL is the absolute lowest TTL we use in CoreDNS.
+	MinimalDefaultTTL = 5 * time.Second
+	// MaximumDefaulTTL is the maximum TTL was use on RRsets in CoreDNS.
+	MaximumDefaulTTL = 1 * time.Hour
+)