RFR: 8325590: Regression in round-tripping UTF-16 strings after JDK-8311906 [v2]

[Roger Riggs]

> Correct the result string coder of a string encoded using a CharsetDecoder with multi-byte encoded input.
> Added tests for UTF16 strings and a regression test.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Test Files.readString with multiple charsets
  Cleanup regression test to match style of other tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17817/files
  - new: https://git.openjdk.org/jdk/pull/17817/files/45ed4e19..39403b00

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17817&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17817&range=00-01

  Stats: 21 lines in 1 file changed: 15 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17817/head:pull/17817

PR: https://git.openjdk.org/jdk/pull/17817