lnwire: encode channel_update type in onion errors

For about a year [1], the spec has prescribed encoding channel_updates
with their type prefix (0x0102) in onion failure messages. LND can
decode correctly with or without the prefix but hasn't been writing the
prefix during encoding. This commit starts writing the prefix.

[1] https://github.com/lightning/bolts/pull/979

lnwire/onion_error_test.go

@@ -184,15 +184,15 @@ func TestWriteOnionErrorChanUpdate(t *testing.T) {
 	// raw serialized length.
 	var b bytes.Buffer
 	update := testChannelUpdate
-	if err := update.Encode(&b, 0); err != nil {
+	trueUpdateLength, err := WriteMessage(&b, &update, 0)
+	if err != nil {
 		t.Fatalf("unable to write update: %v", err)
 	}
-	trueUpdateLength := b.Len()
 
 	// Next, we'll use the function to encode the update as we would in a
 	// onion error message.
 	var errorBuf bytes.Buffer
-	err := writeOnionErrorChanUpdate(&errorBuf, &update, 0)
+	err = writeOnionErrorChanUpdate(&errorBuf, &update, 0)
 	require.NoError(t, err, "unable to encode onion error")
 
 	// Finally, read the length encoded and ensure that it matches the raw