p2p: msg.Payload contains list data

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

p2p/message_test.go

@@ -2,10 +2,12 @@ package p2p
 
 import (
 	"bytes"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 )
@@ -15,11 +17,11 @@ func TestNewMsg(t *testing.T) {
 	if msg.Code != 3 {
 		t.Errorf("incorrect code %d, want %d", msg.Code)
 	}
-	if msg.Size != 5 {
-		t.Errorf("incorrect size %d, want %d", msg.Size, 5)
+	expect := unhex("c50183303030")
+	if msg.Size != uint32(len(expect)) {
+		t.Errorf("incorrect size %d, want %d", msg.Size, len(expect))
 	}
 	pl, _ := ioutil.ReadAll(msg.Payload)
-	expect := []byte{0x01, 0x83, 0x30, 0x30, 0x30}
 	if !bytes.Equal(pl, expect) {
 		t.Errorf("incorrect payload content, got %x, want %x", pl, expect)
 	}
@@ -139,3 +141,11 @@ func TestEOFSignal(t *testing.T) {
 	default:
 	}
 }
+
+func unhex(str string) []byte {
+	b, err := hex.DecodeString(strings.Replace(str, "\n", "", -1))
+	if err != nil {
+		panic(fmt.Sprintf("invalid hex string: %q", str))
+	}
+	return b
+}