requestscopedtype fix being overwritten

When you have an application wide dependency registered and a request based dependency defined in a `IRegistrations` implemnetation it is not getting picked up.

This is because the foreach loop is overriding `RequestScopedTypes` on each iteration here https://github.com/NancyFx/Nancy/blob/master/src/Nancy/Bootstrapper/NancyBootstrapperWithRequestContainerBase.cs#L116

So when it registers the request scoped types here there is nothing to register https://github.com/NancyFx/Nancy/blob/master/src/Nancy/Bootstrapper/NancyBootstrapperWithRequestContainerBase.cs#L157

A repro can be seen here https://github.com/jchannon/nancyrequestproj
I would prefer to see

`this.RequestScopedTypes = Enumerable.Empty<TypeRegistration>();`