Right now, when ATS generates a 304 response (Not Modified), we always remove the Last-Modified header. Reading the RFC, we should only remove the Last-Modified header if there is an ETag header. This is a simple fix, and we should just do it IMO.
It also always removes the Expires headers, which we are not supposed to touch. Now, we do overwrite the Cc: header, so maybe we should remove that too ?
The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).