RFR: 4670339: (ch) Channels.newChannel(OutputStream) should flush stream on every write

[Brian Burkhalter]

Call `flush()` on the `OutputStream` before returning from `WritableByteChannelImpl::write`.

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Commit messages:
 - 4670339: (ch) Channels.newChannel(OutputStream) should flush stream on every write

Changes: https://git.openjdk.org/jdk/pull/13198/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4670339
  Stats: 45 lines in 2 files changed: 40 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13198.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13198/head:pull/13198

PR: https://git.openjdk.org/jdk/pull/13198