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/rlpx_test.go

@@ -3,8 +3,6 @@ package p2p
 import (
 	"bytes"
 	"crypto/rand"
-	"encoding/hex"
-	"fmt"
 	"io/ioutil"
 	"strings"
 	"testing"
@@ -32,7 +30,7 @@ ba628a4ba590cb43f7848f41c4382885
 `)
 
 	// Check WriteMsg. This puts a message into the buffer.
-	if err := EncodeMsg(rw, 8, []interface{}{1, 2, 3, 4}); err != nil {
+	if err := EncodeMsg(rw, 8, 1, 2, 3, 4); err != nil {
 		t.Fatalf("WriteMsg error: %v", err)
 	}
 	written := buf.Bytes()
@@ -68,14 +66,6 @@ func (fakeHash) BlockSize() int              { return 0 }
 func (h fakeHash) Size() int           { return len(h) }
 func (h fakeHash) Sum(b []byte) []byte { return append(b, h...) }
 
-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
-}
-
 func TestRlpxFrameRW(t *testing.T) {
 	var (
 		aesSecret      = make([]byte, 16)
@@ -112,7 +102,7 @@ func TestRlpxFrameRW(t *testing.T) {
 	for i := 0; i < 10; i++ {
 		// write message into conn buffer
 		wmsg := []interface{}{"foo", "bar", strings.Repeat("test", i)}
-		err := EncodeMsg(rw1, uint64(i), wmsg)
+		err := EncodeMsg(rw1, uint64(i), wmsg...)
 		if err != nil {
 			t.Fatalf("WriteMsg error (i=%d): %v", i, err)
 		}