RFR: 8359709: java.net.HttpURLConnection sends unexpected "Host" request header in some cases after JDK-8344190

[Jaikiran Pai]

Can I please get a review for this change which addresses a regression that was introduced in `HttpURLConnection` in Java 24 when we cleaned up the code by removing the references to SecurityManager APIs.

When a HTTP request is issued through `java.net.HttpURLConnection`, then the request URL is used to determine the `Host` header to set in the request. By default, the application cannot set a `Host` header to a different value. However the JDK allows a system property to be enabled to allow applications to explicitly set a `Host` request header when issuing the request.

Due to an oversight in the change that was done in https://bugs.openjdk.org/browse/JDK-8344190, the `Host` header that is set by the application, may not get used for that request causing this regression. Turns out we don't have tests in this area to catch this issue.

The commit in this PR fixes the regression and also introduces a new jtreg test which reproduces the issue and verifies the fix.

I've also checked the original change which introduced this regression https://github.com/openjdk/jdk/pull/22232 to see if there's anything else that needs attention. I haven't stopped anything else.

The

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Commit messages:
 - 8359709: java.net.HttpURLConnection sends unexpected "Host" request header in some cases after JDK-8344190
 - 8359709: introduce a test

Changes: https://git.openjdk.org/jdk/pull/25844/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25844&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359709
  Stats: 145 lines in 2 files changed: 143 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25844.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25844/head:pull/25844

PR: https://git.openjdk.org/jdk/pull/25844