FlushOutGoingMessagesAsync() called before After() Middleware, causing messages not to be sent

[JakeStevenson]

Describe the bug
When applying After() middleware that utilizes the messagebus, messages are not sent.

To Reproduce
Sample Code:

// This works when directly injected into my wolverine asp.net handlers, and clients receive the message
    public class InformAllClients
    {
        private IMessageBus _bus;
        public InformAllClients(IMessageBus bus)
        {
            _bus = bus;
        }
        public async Task Inform(MyAppMessage message)
        {
            await _bus.SendAsync(new UpdateProcessedNotification() { CommandName= message.GetType().Name });
        }
    }

// This does get called when registered as middleware, but clients do not receive the message
// options.Policies.ForMessagesOfType<MyAppMessage>().AddMiddleware(typeof(InformAllClientsMiddleware));
    public static class InformAllClientsMiddleware
    {
        public static async Task After(MyAppMessage message, IMessageBus bus)
        {
            System.Diagnostics.Debug.WriteLine($"Informing {message.GetType().Name}");
            await bus.SendAsync(new UpdateProcessedNotification() { CommandName= message.GetType().Name });
        }
    }

When examining the generated code, we can see this:

        public override async System.Threading.Tasks.Task Handle(Microsoft.AspNetCore.Http.HttpContext httpContext)
        {
            // .... code that does the work
            // Have to flush outgoing messages just in case Marten did nothing because of https://github.com/JasperFx/wolverine/issues/536
            await messageContext.FlushOutgoingMessagesAsync().ConfigureAwait(false);

            await CompounderPlus.Application.Services.InformAllClientsMiddleware.After(command, messageContext).ConfigureAwait(false);
            // Wolverine automatically sets the status code to 204 for empty responses
            if (!httpContext.Response.HasStarted) httpContext.Response.StatusCode = 204;
        }
    }

It appears that calling FlushOutgoingMessagesAsync() before it calls the middleware prevents the messages from being sent.