ASP.NET Core improvements

[vzabrodin]

context.ObjectType can be null in some cases. For example when context.Object is null.
Nullish object type could be passed to MessagePackSerializer.Serialize() method which cause exception.

I have added context.ObjectType == null condition. In this case object type will be taken directly from object: context.Object.GetType()

[neuecc]

Thanks, completely good.