RFR: 8368955: Improve HttpExchange.sendResponseHeaders usability [v3]

[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:

  Update HttpExchange.java

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27613/files
  - new: https://git.openjdk.org/jdk/pull/27613/files/d1d7c612..f78aa328

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=01-02

  Stats: 23 lines in 1 file changed: 3 ins; 5 del; 15 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