Update test for bad ID

The new test sends two replies: the first one has a bad ID, which should
be ignored, and the second one has the correct ID.

client_test.go

@@ -176,12 +176,20 @@ func TestClientSyncBadID(t *testing.T) {
 	m.SetQuestion("miek.nl.", TypeSOA)
 
 	c := new(Client)
-	if _, _, err := c.Exchange(m, addrstr); err != ErrId {
-		t.Errorf("did not find a bad Id")
+	r, _, err := c.Exchange(m, addrstr)
+	if err != nil {
+		t.Errorf("failed to exchange: %v", err)
+	}
+	if r.Id != m.Id {
+		t.Errorf("failed to get response with expected Id")
 	}
 	// And now with plain Exchange().
-	if _, err := Exchange(m, addrstr); err != ErrId {
-		t.Errorf("did not find a bad Id")
+	r, err = Exchange(m, addrstr)
+	if err != nil {
+		t.Errorf("failed to exchange: %v", err)
+	}
+	if r.Id != m.Id {
+		t.Errorf("failed to get response with expected Id")
 	}
 }
 

server_test.go

@@ -33,6 +33,9 @@ func HelloServerBadID(w ResponseWriter, req *Msg) {
 	m.Extra = make([]RR, 1)
 	m.Extra[0] = &TXT{Hdr: RR_Header{Name: m.Question[0].Name, Rrtype: TypeTXT, Class: ClassINET, Ttl: 0}, Txt: []string{"Hello world"}}
 	w.WriteMsg(m)
+
+	m.Id--
+	w.WriteMsg(m)
 }
 
 func HelloServerEchoAddrPort(w ResponseWriter, req *Msg) {