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.
------------- Commit messages: - Improve HttpExchange.sendResponseHeaders usability Changes: https://git.openjdk.org/jdk/pull/27613/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8368955 Stats: 20 lines in 1 file changed: 12 ins; 0 del; 8 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
