RFR: 8359709: java.net.HttpURLConnection sends unexpected "Host" request header in some cases after JDK-8344190 [v2]

[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.

Jaikiran Pai has updated the pull request incrementally with two additional commits since the last revision:

 - Volkan's suggestion - use test specific context root for the handler
 - Daniel's suggestion - use setIfNotSet

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25844/files
  - new: https://git.openjdk.org/jdk/pull/25844/files/6dc36858..666f29b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25844&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25844&range=00-01

  Stats: 8 lines in 2 files changed: 1 ins; 1 del; 6 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