Redirects are not cached for wp_old_slug_redirect

Steps to reproduce:
1. Create a post with date day `x`.
2. Update post with date day `y`
3. Visit old url with date of `x`, observe headers in redirect.

The `Location` is added correctly, however there is no `max-age` in the `Cache-Control` and no `X-Batcache: HIT` status. I've created https://github.com/humanmade/batcache/pull/21 to fix that. Previously `cache_redirects` would not work if `Cache-Control: max-age=0` is also sent, which is often the case.