RFR: 8349135: Add tests for HttpRequest.Builder.copy()

[Volkan Yazici]

Adds `HttpRequest.Builder::copy` tests to `HttpRequestBuilderTest`.

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Commit messages:
 - Verify `copy()` in `HttpRequestBuilderTest`

Changes: https://git.openjdk.org/jdk/pull/23415/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349135
  Stats: 37 lines in 1 file changed: 37 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23415/head:pull/23415

PR: https://git.openjdk.org/jdk/pull/23415