Resolves #20 - NullReferenceException when HttpContext.Current is null

[craigfowler]

This pull request will resolve #20 - RequestLifetimeScopeProvider.EndLifetimeScope can throw NullReferenceException if HttpContext.Current is null (such as when the HTTP request makes use of only OWIN middleware and does not pass through the regular MVC stack).

[tillig]

Would it be better to put the HttpContext.Current check in the LifetimeScope getter/setter? That's currently the only property that touches HttpContext.Current and it looks like EndLifetimeScope checks for a null scope before trying to dispose of it.

[craigfowler]

@tillig - I did consider that, but then we'd end up with a property for which its get/set are not parallel in their semantics.

Sure, when HttpContext.Current is null, then the getter would return null, but the setter would logically have to do one of the two following:

• Raise an exception if anything tries to set it and HttpContext.Current is null (as it would do now)
• Silently become a no-op if anything tries to set it and HttpContext.Current is null

I considered introducing that discrepancy to be a greater evil than introducing a duplicate check on the HttpContext.Current elsewhere.

That said - it's a borderline case, so I'm happy if you'd prefer it pushed into the property getter/setter. An alternative is to do away with that static property altogether and replace it with separate get/set methods, which might make it clear that it's not expected to "just behave like a normal C# property".

[tillig]

I do see this line in GetLifetimeScope is actually also using a specific HttpContext.Current check so maybe checking in the EndLifetimeScope is fine. I'm not sure I feel strongly about it one way or the other. Given it's all internal I suppose we could refactor it later if needed.