RFR: 8276207: Properties.loadFromXML/storeToXML works incorrectly for supplementary characters

[Anirvan Sarkar]

This fixes Properties.loadFromXML/storeToXML so that it works correctly for supplementary characters.

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Commit messages:
 - 8276207: Properties.loadFromXML/storeToXML works incorrectly for supplementary characters

Changes: https://git.openjdk.java.net/jdk/pull/6216/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6216&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276207
  Stats: 90 lines in 3 files changed: 58 ins; 16 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6216.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6216/head:pull/6216

PR: https://git.openjdk.java.net/jdk/pull/6216