[web] fix headers on 304 responses (closes #11466875) The response MUST include the following header fields: [...] - ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5 Unfortunately we weren't copying headers from what would have been the 200 response, but from the request.

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