RFR: 8326949: Authorization header is removed when a proxy Authenticator is set on HttpClient

[Michael McMahon]

This fix relaxes the constraints on user set authentication headers. Currently, any user set authentication headers are filtered out, if the HttpClient has an Authenticator set. The reason being that the authenticator is expected to manage authentication.  With this fix, it will be possible to use pre-emptive authentication through user set headers, even if an authenticator is set. The expected use case is where the authenticator would manage either proxy or server authentication and the user set headers would manage server authentication if the authenticator is managing proxy (or vice versa).
If the pre-emptive authentication fails, then this behavior is disabled on further retries and it would be up to the authenticator to provide the right credentials then.

Thanks,
Michael

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Commit messages:
 - test update
 - test update
 - test update
 - test update
 - initial impl with test
 - Merge branch 'master' into 8326949-authorize
 - impl for fix

Changes: https://git.openjdk.org/jdk/pull/21249/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21249&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326949
  Stats: 376 lines in 6 files changed: 364 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/21249.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21249/head:pull/21249

PR: https://git.openjdk.org/jdk/pull/21249