RFR: 8329662: Add a test to verify the behaviour of the default HEAD() method on HttpRequest.Builder

[Jaikiran Pai]

Can I please get a review of this test-only change which updates an existing test case to verify the behaviour of `HttpRequest.Builder.HEAD()` method?

As noted in https://bugs.openjdk.org/browse/JDK-8329662, this test now verifies the default implementation specified for `HttpRequest.Builder.HEAD()`. The existing tests were only verifing the behaviour of the `HEAD()` method implementation in the class of the instance returned from `HttpRequest.newBuilder()`. 

I have run this test in our CI with these changes and the test continues to pass.

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Commit messages:
 - 8329662: Add a test to verify the behaviour of the default HEAD() method on HttpRequest.Builder

Changes: https://git.openjdk.org/jdk/pull/18622/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18622&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329662
  Stats: 86 lines in 1 file changed: 85 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18622.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18622/head:pull/18622

PR: https://git.openjdk.org/jdk/pull/18622