LegacyOAuthSecurityTokenHandler - Expired Token

Hi

We had an issue with the example LegacyOAuthSecurityTokenHandler and it not checking for Expired tokens. We changed it to the below which resolved the issue for us.

```
            var ticket = LegacyOAuthSecurityTokenHelper.GetTicket(token, _options);

            if (ticket.Properties.ExpiresUtc < DateTime.UtcNow)
                throw new SecurityTokenExpiredException();
```