improve the HttpExchange api with documented constants and convenience methods to avoid common bugs
------------- Commit messages: - improve HttpExchange response api Changes: https://git.openjdk.org/jdk/pull/18955/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18955&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8331195 Stats: 48 lines in 1 file changed: 48 ins; 0 del; 0 mod Patch: https://git.openjdk.org/jdk/pull/18955.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/18955/head:pull/18955 PR: https://git.openjdk.org/jdk/pull/18955
