verify method
Determines whether a cookie is properly signed, if it is signed at all.
If there is no signature, returns false
.
If the provided signature does not match the payload
provided, returns false
.
Otherwise, returns true.
Implementation
bool verify(Cookie cookie) {
var data = getCookiePayloadAndSignature(cookie.value);
return (data != null && (data[1] == computeCookieSignature(data[0])));
}