Fix #30 - only status is required to bypass not-found interceptor

Here is a proposed solution to #30.

I'm not happy that we'd be replicating part of [ring.util.response/response?](https://github.com/mmcgrana/ring/blob/master/ring-core/src/ring/util/response.clj#L152), but the amount of code is fairly minimal and it is checking something subtly different. As @timewald said, just a status code should be sufficient for a valid response. We're checking for a valid response, whereas `response?` is checking for a valid response **map**.