RetentionUntilDate in SetObjectRetentionArgs serialized to wrong value

[basrach]

When RetentionUntilDate property in SetObjectRetentionArgs class serialized, it formatted to ISO 8601 string, but time zone offset get lost.

public static string To8601String(DateTime dt)
{
    return dt.ToString("yyyy-MM-dd'T'HH:mm:ssZ", CultureInfo.InvariantCulture);
}

As result if date in local time zone, minio server received date in past or future depending on time zone.
I tried pass date in UTC, but get error, because passed value checked against local time (DateTime.Now) always

if (DateTime.Compare(RetentionUntilDate, DateTime.Now) <= 0)
            throw new InvalidOperationException("Retention until date set using " + nameof(WithRetentionUntilDate) +
                                                " needs to be in the future.");