Issue/7

This fixes the `content-type` being set to `application/octet-stream` for dynamic content.

We:
 - let the `content-type` to `undefined` if we are not able to parse the url and lookup for a valid one. Another middleware or the user will be responsible to set a valid `content-type`.
 - force `refresh` as we don't know if the user wants to cache.

Diff wo/ spaces: [#10](https://github.com/h5bp/node-server-config/pull/10?w=0).
Related to #7.