RFR: The latest request may not contain the full set of headers

[Robin Westberg]

When returning a cached result as a response to a 304, use the combined set of headers.

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Commit messages:
 - The latest request may not contain the full set of headers

Changes: https://git.openjdk.java.net/skara/pull/972/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=972&range=00
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/972.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/972/head:pull/972

PR: https://git.openjdk.java.net/skara/pull/972