Cause: An attempt is made to freshen a stale entry in the cache, and the 304 Not Modified response from the backend contains no ETag, no Last-Modified, and no Cache-Control header.
Consequence: Because the 304 response from backend is uncacheable, httpd sends the cached response back, but without the original headers. This can lead to situation when for example Content-Type of cached response is not sent and therefore lost.
Fix: Headers of cached original response are sent in the response.
Result: httpd returns proper headers in this situation.