htlcswitch/hop: always accept even custom tlv records

This commit prepares for sending and receiving of application-specific
custom tlv fields.

htlcswitch/hop/payload_test.go

@@ -130,6 +130,12 @@ var decodePayloadTests = []decodePayloadTest{
 			FinalHop:  false,
 		},
 	},
+	{
+		name: "required type in custom range",
+		payload: []byte{0x02, 0x00, 0x04, 0x00,
+			0xfe, 0x00, 0x01, 0x00, 0x00, 0x00,
+		},
+	},
 	{
 		name: "valid intermediate hop",
 		payload: []byte{0x02, 0x00, 0x04, 0x00, 0x06, 0x08, 0x01, 0x00,