Owin Extensions in F# - second review

Constants.fs
* Is there a reason to have RequestFormKey different from the others?
* I'm not sure about the visibility of these outside of the assembly

EnvironmentExtensions.fs
* The Helpers module is only there so that ParseForm can be visible for the unit test Otherwise, I would just make it a `let` inside the type.
* I don't think the type hosting the extension methods matter, so I named it `x` to keep it short since I have to reference it a lot internally
* I wanted to make these instance methods, as referenced in http://stackoverflow.com/questions/702256/f-extensions-in-c-sharp however it seems that I can't do that with constrained generic types such as `IDictionary<'string,'Object>`.