Do not set headers in case of directory, no index and passthrough

When running _st_ as middleware with `next` function, `index: false`, and `passthrough: true`, still cache cache headers are being set. It shouldn't be the case as request is expected to be handled outside of _st_

Today I run into issues, when main page of my application was not refreshed by the browser, and user after login was still seing public page, it was caused by `Cache-control: public` (as set by _st_) and browser didn't try to refetch the page.