RFR: 8349135: Add tests for HttpRequest.Builder.copy() [v4]

[Volkan Yazici]

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

Volkan Yazici has updated the pull request incrementally with one additional commit since the last revision:

  Replace `assert`s with explicit checks throwing `AssertionError`s

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23415/files
  - new: https://git.openjdk.org/jdk/pull/23415/files/4e3b9a67..5eddbf7d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23415&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23415&range=02-03

  Stats: 29 lines in 1 file changed: 8 ins; 11 del; 10 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