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/cache.go

@@ -74,7 +74,7 @@ func (c *CachingResponseWriter) WriteMsg(res *dns.Msg) error {
 
 func (c *CachingResponseWriter) set(m *dns.Msg, key string, mt middleware.MsgType) {
 	if key == "" {
-		// logger the log? TODO(miek)
+		log.Printf("[ERROR] Caching called with empty cache key")
 		return
 	}
 
@@ -94,6 +94,10 @@ func (c *CachingResponseWriter) set(m *dns.Msg, key string, mt middleware.MsgTyp
 		i := newItem(m, duration)
 
 		c.cache.Set(key, i, duration)
+	case middleware.OtherError:
+		// don't cache these
+	default:
+		log.Printf("[WARNING] Caching called with unknown middleware MsgType: %d", mt)
 	}
 }