RFR: 8349664: Certificate.java toString HEX dump should always use ASCII or ISO_8859_1

[Mikhail Yankelevich]

Changed `HexDumpEncoder`, so `encodeBuffer` always uses ISO_8859_1. 
This also fixes the issue with conversion in `encode` method, as the input is always ISO_8859_1.

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Commit messages:
 - 8349664: Certificate.java toString HEX dump should always use ASCII or ISO_8859_1

Changes: https://git.openjdk.org/jdk/pull/23661/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23661&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349664
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23661.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23661/head:pull/23661

PR: https://git.openjdk.org/jdk/pull/23661