RFR: 6968351: httpserver clashes with delayed TCP ACKs for low Content-Length [v19]

[robert engels]

> fix bug JDK-B6968351 by avoiding flush after response headers

robert engels has updated the pull request incrementally with one additional commit since the last revision:

  clean and move test based on PR review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18667/files
  - new: https://git.openjdk.org/jdk/pull/18667/files/207a3a92..3fe66dc3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18667&range=18
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18667&range=17-18

  Stats: 241 lines in 2 files changed: 124 ins; 117 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18667.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18667/head:pull/18667

PR: https://git.openjdk.org/jdk/pull/18667