RFR: 8325361: Make sun.net.www.MessageHeader final

[Korov]

Make sun.net.www.MessageHeader final, and replace C-style array type declaration with Java-style array declaration.

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Commit messages:
 - 8325361: Make sun.net.www.MessageHeader final

Changes: https://git.openjdk.org/jdk/pull/17997/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17997&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325361
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17997.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17997/head:pull/17997

PR: https://git.openjdk.org/jdk/pull/17997