如何在massTransit 4.X上设置邮件头

Question

当我在massTransit 3.x中时，我使用follow代码在发送之前设置消息头，它的工作。

public class SendObserver : ISendObserver
{
    public Task PostSend<T>(SendContext<T> context) where T : class
    {
        return Task.FromResult<object>(null);
    }

    public Task PreSend<T>(SendContext<T> context) where T : class
    {
        //1. get current thread identity object.
        var currentThreadIdentityObject = Thread.CurrentPrincipal.Identity is JustIMIdentity ? (Thread.CurrentPrincipal.Identity as JustIMIdentity).IdentityObject : null;

        if (currentThreadIdentityObject != null)
        {
            //2. set the headers.
            context.Headers.Set(AppSettingConfig.RabbitMqIdentityHeaderKey, currentThreadIdentityObject.StaffID);
        }

        return Task.FromResult<object>(null);
    }

    public Task SendFault<T>(SendContext<T> context, Exception exception) where T : class
    {
        return Task.FromResult<object>(null);
    }
}

我在autofac中注册

        //register in autofac
        builder.Register(context =>
        {
            var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
            {
                var uri = AppSettingConfig.RabbitMqBaseUri;

                var host = cfg.Host(new Uri(uri), x =>
                {
                    x.Username(AppSettingConfig.RabbitMqUserName);
                    x.Password(AppSettingConfig.RabbitMqUserPassword);
                });
            });

            busControl.ConnectSendObserver(new SendObserver());

            busControl.Start();

            return busControl;
        })
        .SingleInstance()
        .As<IBusControl>()
        .As<IBus>();

        builder.RegisterGeneric(typeof(RequestClient<,>)).AsSelf().AsImplementedInterfaces();

但是当我将massTransit更新为4.X时，队列中的消息没有标题，看起来像set header不起作用，是怎么回事。