Turn off output buffering, allowing Nancy to control it

Under ASP.NET hosting, output buffering is enabled by default and must be disabled on its Response object if it isn't to be used (regardless of any IIS settings).

Unless this is disabled at this point (since there is no way to control it at runtime), output will always be buffered and ultimately exhaust available memory when dealing with large outputs (i.e. streamed file downloads using GenericFileResponse).