public SessionAWSCredentials(string awsAccessKeyId, string awsSecretAccessKey, string token)
{
if (string.IsNullOrEmpty(awsAccessKeyId)) throw new ArgumentNullException("awsAccessKeyId");
if (string.IsNullOrEmpty(awsSecretAccessKey)) throw new ArgumentNullException("awsSecretAccessKey");
if (string.IsNullOrEmpty(token)) throw new ArgumentNullException("token");
_lastCredentials = new ImmutableCredentials(awsAccessKeyId, awsSecretAccessKey, token);
}