RFR: 8263552: Use String.valueOf() for char-to-String conversion in ObjectStreamClass

[Сергей Цыпанов]

This is a very simple and trivial improvement about getting rid of pointless char wrapping into array

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Commit messages:
 - Fix other occurences
 - Use String.valueOf() for char-to-String conversion in ObjectStreamClass

Changes: https://git.openjdk.java.net/jdk/pull/2660/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2660&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263552
  Stats: 6 lines in 6 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2660.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2660/head:pull/2660

PR: https://git.openjdk.java.net/jdk/pull/2660