middleware/caching: don't set TTL on OPT

When setting the TTL on all RR in the message we would also do this
for the OPT RR. This is wrong as the OPT RR does *not* have a TTL.

middleware/cache/item.go

@@ -74,6 +74,9 @@ func setCap(m *dns.Msg, ttl uint32) {
 		r.Header().Ttl = uint32(ttl)
 	}
 	for _, r := range m.Extra {
+		if r.Header().Rrtype == dns.TypeOPT {
+			continue
+		}
 		r.Header().Ttl = uint32(ttl)
 	}
 }