Update cache_expires

Hi @namusyaka, is there any particular reason why `cache_expires_in` isn't `cache_expires`? Padrino is using the latter. See [here](https://github.com/padrino/padrino-framework/blob/master/padrino-core/lib/padrino-core/ext/http_router.rb#L48) and [here](https://github.com/padrino/padrino-framework/blob/master/padrino-cache/lib/padrino-cache/helpers/page.rb#L63). Thanks :)