RFR: 8267529: StringJoiner can create a String that breaks String::equals

[Claes Redestad]

This patch fixes a regression caused by https://bugs.openjdk.java.net/browse/JDK-8265237 where the result of String.join always get a UTF-16 coder if the delimiter is UTF-16, even when no delimiter is emitted.

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Commit messages:
 - Fix copyright, @bug
 - 8267529: StringJoiner can create a String that breaks String::equals

Changes: https://git.openjdk.java.net/jdk/pull/4228/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4228&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267529
  Stats: 18 lines in 2 files changed: 14 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4228.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4228/head:pull/4228

PR: https://git.openjdk.java.net/jdk/pull/4228