Include HTTP-caching headers to "304 Not Modified" responses

[Rinat Khabibiev]

Most browsers can cache responses with 304 HTTP code. So why not to include such headers to response if necessary.

UPDATE:
According to the ​RFC7232 all "304 Not Modified" responses must include "Cache-Control" and "Expires" headers that would have been sent in a corresponding "200 OK" response.

See ​Section 4.1 of RFC7232

[Tim Graham]

I didn't spot a reason for the behavior in the original ticket (#580). Can you motivate this change with an example? You say "most browsers can cache responses with 304 HTTP code" -- any idea why all browsers don't?

[Rinat Khabibiev]

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.

Taken from RFC7232: ​https://tools.ietf.org/html/rfc7232#section-4.1.

Usually responses with 200 code will include such headers (e.g. from views decorated with cache_page), so according to the RFC responses with 304 code must also include them.