I would like to see a feature to enable proxy_cache re-validation (proxy_cache_revalidate on|off?)

When cached content is expired, nginx should check with the origin server if the content has changed.
This to prevent unnecessary downloads from the origin if the content was not changed.

e.g.

If configured:
proxy_cache_valid $value
proxy_cache_revalidate on

If content is expired after $value, re-validate with origin, if content has not been changed (304), extend the validation with proxy_cache_valid value.
When the content is changed, fetch and cache new data from origin.