middleware/cache: don't cache expired RRSIGs (#641)

Check message for expired sig and don't cache those.

Aside: This hack of caching entire messages is probably something we
should stop doing at some point in the future and do this on a per RRset
basis.

Fixes #367 #635

middleware/cache/cache.go

@@ -63,7 +63,7 @@ type ResponseWriter struct {
 // WriteMsg implements the dns.ResponseWriter interface.
 func (c *ResponseWriter) WriteMsg(res *dns.Msg) error {
 	do := false
-	mt, opt := response.Typify(res)
+	mt, opt := response.Typify(res, time.Now().UTC())
 	if opt != nil {
 		do = opt.Do()
 	}