[BUG] Incorrect behaviors registration with assemblies scanning

[quatrebi]

Built-in RequestPre/PostProcessorBehaviors are not registered in DI after scaning assemblies for IRequestPre/PostProcessor.

Actual

I have MediatR injection with the following config:

        services.AddMediatR(cfg =>
        {
            cfg.RegisterServicesFromAssemblies(assembliesToScan);
            cfg.AutoRegisterRequestProcessors = true;
        });

and my Pre/PostProcessor are rigistered succesfully, but not behaviour for that.

So, I go deep into underhood MediatR and found following:

• Pre/PostProcessors will be registered if AutoRegisterRequestProcessors == true . source code *link*
• RequestPre/PostProcessorBehavior will be registerd if we have at least one ServiceDescriptor in MediatR configuration!!: cfg.RequestPreProcessorsToRegister.Any() == true. source code *link*

Expected

Built-in RequestPre/PostProcessorBehaviors also MUST BE registred when any Pre/PostProcessers have already been registered