public async Task WithDeadline()
        {
            var now = DateTime.UtcNow.AddMinutes(10);
            var opt = (CallOptions)client.AsDynamic().option;

            opt.Deadline.IsNull();

            var change = client.WithDeadline(now);

            opt = (CallOptions)change.AsDynamic().option;
            opt.Deadline.Is(now);

            var context = await change.ReturnContext();

            context.Deadline.ToString().Is(now.ToString()); // almost same:)
        }