RFR: 8305920 : Replace String concatenation in a loop in SimpleAttributeSet.toString with StringBuilder

[Renjith Kannath Pariyangad]

Replaced string concatenation using '+' operator with StringBuilder append function.

Please review.

Regards,
Renjith.

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Commit messages:
 - Updated copyright year
 - JDK-8305920 : Replace String concatenation in a loop in SimpleAttributeSet.toString with StringBuilder

Changes: https://git.openjdk.org/jdk/pull/13479/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13479&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305920
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13479/head:pull/13479

PR: https://git.openjdk.org/jdk/pull/13479