RFR: 8331195: Improve com.sun.net.httpserver.HttpExchange usability

[robert engels]

improve the HttpExchange api with documented constants and convenience methods to avoid common bugs

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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