Populated NegotiationContext when NancyContext is created

If you have a `StatusCodeHandler` that responds to exceptions and returns a negotiated response the `NegotiationContext` is null when an exception occurs before a module is built.  `NegotiationContext` is needed in the  `WithModel` method. The `DefaultNancyModuleBuilder` is where it currently only gets set. I have set the `NegotiationContext` now in `DefaultNancyContextFactory` when the `NancyContext` is created.