Fix ExamplePrivateHandle (miekg#1354)

Signed-off-by: Miek Gieben <miek@miek.nl>

example_test.go

@@ -121,21 +121,23 @@ func (rd *APAIR) Len() int {
 func ExamplePrivateHandle() {
 	dns.PrivateHandle("APAIR", TypeAPAIR, NewAPAIR)
 	defer dns.PrivateHandleRemove(TypeAPAIR)
+	var oldId = dns.Id
+	dns.Id = func() uint16 { return 3 }
+	defer func() { dns.Id = oldId }()
 
 	rr, err := dns.NewRR("miek.nl. APAIR (1.2.3.4    1.2.3.5)")
 	if err != nil {
 		log.Fatal("could not parse APAIR record: ", err)
 	}
-	fmt.Println(rr)
-	// Output: miek.nl.	3600	IN	APAIR	1.2.3.4 1.2.3.5
+	fmt.Println(rr) // see first line of Output below
 
 	m := new(dns.Msg)
-	m.Id = 12345
 	m.SetQuestion("miek.nl.", TypeAPAIR)
 	m.Answer = append(m.Answer, rr)
 
 	fmt.Println(m)
-	// ;; opcode: QUERY, status: NOERROR, id: 12345
+	// Output: miek.nl.	3600	IN	APAIR	1.2.3.4 1.2.3.5
+	// ;; opcode: QUERY, status: NOERROR, id: 3
 	// ;; flags: rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
 	//
 	// ;; QUESTION SECTION: