RFR: 8357013: Optimize response code parsing in HttpURLConnection

[Patrick Strawderman]

Avoid allocating a substring to parse the response code in HttpURLConnection#getResponseCode, but instead use the Integer.parseInt overload that accepts String indices.

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Commit messages:
 - Optimize response code parsing in HttpURLConnection

Changes: https://git.openjdk.org/jdk/pull/25245/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25245&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357013
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25245/head:pull/25245

PR: https://git.openjdk.org/jdk/pull/25245