mod_cache_socache: Keep the body when a revalidated entry is refreshed.

[arturobernalg]

A 304 response updates the headers of the stored response, but store_headers() rebuilt the entry without the body recalled by open_entity(), so later requests were answered from the cache with the headers and an empty body. Copy the body into the new entry, or drop the entry when it no longer fits.

PR 62493.

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• This PR does not report or address a security vulnerability
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
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