RFR: 8368955: Improve HttpExchange.sendResponseHeaders usability [v2]

[Josiah Noel]

> As specified in [JDK-8368955](https://bugs.openjdk.org/browse/JDK-8368955?jql=project%20in%20(JDK)%20AND%20component%20in%20(core-libs)%20AND%20Subcomponent%20in%20(java.net)), adds some constants to HttpExchange to make `sendResponseHeaders` more usable.

Josiah Noel has updated the pull request incrementally with one additional commit since the last revision:

  pr comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27613/files
  - new: https://git.openjdk.org/jdk/pull/27613/files/3a4de5c5..d1d7c612

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27613.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27613/head:pull/27613

PR: https://git.openjdk.org/jdk/pull/27613