Resolve cyclic dependencies between Http.Results and Http.Extensions

[captainsafia]

To support users being able to determine if the handler they are wrapping a filter is a void-returning handler, we need to add support for returning the EmptyHttpResult from the filter pipeline. However, do to cyclic dependencies we cannot use the defined EmptyHttpResult type. This prevents users from being able to assert on the type.

[davidfowl]

One thing we can do is move results down and then typeforward them.