SignedRequestHelper2 constructor
Implementation
SignedRequestHelper2(String email, String secretKey) {
if (secretKey == null) throw ArgumentError("SecretKey cannot be null");
accessString = "email=" + percentEncodeRfc3986(email.toLowerCase()) + "&";
var key = utf8.encode(secretKey);
hmacSha256 = Hmac(sha256, key); // HMAC-SHA256
}