do not modify dns.Rcode when packing to wire format (#675)

* do not modify dns.Rcode when packing to wire format

When the message has an EDNS0 option in the additional section and
dns.Msg.Rcode is set to an extended rcode, dns.Msg.PackBuffer() modifies
dns.Msg.Rcode.
If you were to `Pack` the message and log it after, the Rcode would show
NOERROR.

Running the test before the change would error with:

```
=== RUN   TestPackNoSideEffect
--- FAIL: TestPackNoSideEffect (0.00s)
	msg_test.go:51: after pack: Rcode is expected to be BADVERS
```

after fixing dns.Msg.PackBuffer(), all tests are still passing.

Fixes #674

* address comments from PR#675

msg.go

@@ -714,12 +714,11 @@ func (dns *Msg) PackBuffer(buf []byte) (msg []byte, err error) {
 			return nil, ErrExtendedRcode
 		}
 		opt.SetExtendedRcode(uint8(dns.Rcode >> 4))
-		dns.Rcode &= 0xF
 	}
 
 	// Convert convenient Msg into wire-like Header.
 	dh.Id = dns.Id
-	dh.Bits = uint16(dns.Opcode)<<11 | uint16(dns.Rcode)
+	dh.Bits = uint16(dns.Opcode)<<11 | uint16(dns.Rcode&0xF)
 	if dns.Response {
 		dh.Bits |= _QR
 	}

msg_test.go

@@ -26,6 +26,28 @@ var (
 	})
 )
 
+func TestPackNoSideEffect(t *testing.T) {
+	m := new(Msg)
+	m.SetQuestion(Fqdn("example.com."), TypeNS)
+
+	a := new(Msg)
+	o := &OPT{
+		Hdr: RR_Header{
+			Name:   ".",
+			Rrtype: TypeOPT,
+		},
+	}
+	o.SetUDPSize(DefaultMsgSize)
+
+	a.Extra = append(a.Extra, o)
+	a.SetRcode(m, RcodeBadVers)
+
+	a.Pack()
+	if a.Rcode != RcodeBadVers {
+		t.Errorf("after pack: Rcode is expected to be BADVERS")
+	}
+}
+
 func TestUnpackDomainName(t *testing.T) {
 	var cases = []struct {
 		label          string