New default: the template digest is automatically included in your ETags.
When you call `fresh_when @post`, the digest for `posts/show.html.erb`
is mixed in so future changes to the HTML will blow HTTP caches for you.
This makes it easy to HTTP-cache many more of your actions.

If you render a different template, you can now pass the `:template`
option to include its digest instead:

  fresh_when @post, template: 'widgets/show'

Pass `template: false` to skip the lookup. To turn this off entirely, set:

  config.action_controller.etag_with_template_digest = false