RFR: 8167252: Some of Charset.availableCharsets() does not contain itself

[Naoto Sato]

Adding themselves into their `contains()` method will fix it.

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Commit messages:
 - 8167252: Some of Charset.availableCharsets() does not contain itself

Changes: https://git.openjdk.org/jdk/pull/14473/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14473&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8167252
  Stats: 34 lines in 4 files changed: 28 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/14473.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14473/head:pull/14473

PR: https://git.openjdk.org/jdk/pull/14473