FormsAuthentication Login/logout methods assume configuration is not null

[Worthaboutapig]

The (public) methods to Nancy.Authentication.Forms.FormsAuthentication login and logout methods, e.g. UserLoggedInRedirectResponse(..) or LogOutAndRedirectResponse(...) assume that the FormsAuthenticationConfiguration currentConfiguration has been set by the Enable method.

However, if it's not, then BuildCookie throws a NullReferenceException at runtime, with no explanation of what's wrong. It's taken me about an hour to track down that I'm returning from the bootstrapper RequestStartup before setting the forms configuration. It's a silly mistake, but easy to make.

Would there be an objection to adding a guard on these methods, something like:

if (configuration == null)
{
throw new InvalidOperationException("The internal FormsAuthenticationConfiguration has not been set. Ensure that FormsAuthentication has been enabled in the bootstrapper.");
}

[Crisfole]

+1 for anything that makes tracking down mysterious errors more obvious

[Worthaboutapig]

I've made the changes and created a Pull Request.