serialize method
Implementation
String serialize(Hmac hmac) {
var headerString = base64Url.encode(json.encode(_header).codeUnits);
var payloadString = base64Url.encode(json.encode(toJson()).codeUnits);
var data = '$headerString.$payloadString';
var signature = hmac.convert(data.codeUnits).bytes;
return '$data.${base64Url.encode(signature)}';
}