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

[Korov]

Make sun.net.www.MessageHeader final, and remove `@SuppressWarnings("this-escape")`

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/18060/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325361
  Stats: 6 lines in 1 file changed: 0 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18060.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18060/head:pull/18060

PR: https://git.openjdk.org/jdk/pull/18060