RFR: 6968351: httpserver clashes with delayed TCP ACKs for low Content-Length [v7]

[robert engels]

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

robert engels has updated the pull request incrementally with two additional commits since the last revision:

 - avoid sending multiple empty chunks
 - test both fixed and chunked responses

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18667/files
  - new: https://git.openjdk.org/jdk/pull/18667/files/aca4cb11..603b093a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18667&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18667&range=05-06

  Stats: 30 lines in 2 files changed: 26 ins; 1 del; 3 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