Include ETag and Last-Modified in 304 response

`ETag` is mandated by https://tools.ietf.org/html/rfc7232#section-4.1:

> 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.

`Last-Modified` is suggested to be included as well.