payload: change signature Decode

To provide backwards compatibility, the Decode function in sphinx.go has
been modified to accept an optional isMessage parameter. This is now not
a breaking change.

Author: gijswijs

payload.go

@@ -89,7 +89,11 @@ func (hp *HopPayload) Encode(w io.Writer) error {
 // Decode unpacks an encoded HopPayload from the passed reader into the target
 // HopPayload. The isMessage boolean should be set to true if we're parsing a
 // payload that is known to be for an onion message.
-func (hp *HopPayload) Decode(r io.Reader, isMessage bool) error {
+func (hp *HopPayload) Decode(r io.Reader, isMessage ...bool) error {
+	// To preserve backwards compatibility, we'll default to isMessage being
+	// false if it is not provided.
+	isMsg := len(isMessage) > 0 && isMessage[0]
+
 	bufReader := bufio.NewReader(r)
 
 	// In order to properly parse the payload, we'll need to check the
@@ -101,7 +105,7 @@ func (hp *HopPayload) Decode(r io.Reader, isMessage bool) error {
 	}
 
 	// Per BOLT 7, onion messages MUST use the TLV format.
-	if isMessage {
+	if isMsg {
 		return decodeTLVHopPayload(hp, bufReader)
 	}