feature request stale_while_revalidate

[enthusiasmus]

Hi guys!

We currently use rack-cache in production and we are quite happy with it.
We discovered the stale_while_revalidate flag for expires_in in rails (https://api.rubyonrails.org/classes/ActionController/ConditionalGet.html#method-i-expires_in).

Is there any chance rack-cache could support something like this?
Returning the cache hit, but also forwarding the request to the rails app for revalidation.

Thanks in advance,
best regards,
Lukas

[grosser]

sounds reasonable ... PR welcome ... just make sure it does not add overhead for the default usecase :)