RFR: 8299015: Ensure that HttpResponse.BodySubscribers.ofFile writes all bytes

[Chris Hegarty]

The implementation of `BodySubscribers.ofFile` does not correctly handle the case of a short gathering write. The write should be retried until all the bytes have been written to the underlying file.

This is generally not an issue that would occur very frequently, since the `IOV_MAX` is commonly 1024 on Mac/Linux, and 16 on Windows. However, it is clearly a bug, and should be fixed.

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Commit messages:
 - Initial changes

Changes: https://git.openjdk.org/jdk/pull/11722/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11722&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299015
  Stats: 35 lines in 2 files changed: 33 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11722.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11722/head:pull/11722

PR: https://git.openjdk.org/jdk/pull/11722