RFR: 8353949: HttpHeaders.firstValueAsLong unnecessarily boxes to Long

[Volkan Yazici]

Avoid unnecessary boxing in `HttpHeaders::firstValueAsLong`.

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Commit messages:
 - Avoid unnecessary boxing in `HttpHeaders::firstValueAsLong`

Changes: https://git.openjdk.org/jdk/pull/24561/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353949
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24561.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24561/head:pull/24561

PR: https://git.openjdk.org/jdk/pull/24561