RFR: 8351368: RichTextArea: exception pasting from Word

[Andy Goryachev]

The original code used String attribute keys.

I'd like to backport this fix to jfx24u.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jfx/pull/1731/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1731&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351368
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1731.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1731/head:pull/1731

PR: https://git.openjdk.org/jfx/pull/1731