RFR: 8331195: Improve com.sun.net.httpserver.HttpExchange usability [v2]

[robert engels]

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

robert engels has updated the pull request incrementally with four additional commits since the last revision:

 - Update src/jdk.httpserver/share/classes/com/sun/net/httpserver/HttpExchange.java
   
   Co-authored-by: Michael McMahon <70538289+Michael-Mc-Mahon at users.noreply.github.com>
 - Update src/jdk.httpserver/share/classes/com/sun/net/httpserver/HttpExchange.java
   
   Co-authored-by: Michael McMahon <70538289+Michael-Mc-Mahon at users.noreply.github.com>
 - Update src/jdk.httpserver/share/classes/com/sun/net/httpserver/HttpExchange.java
   
   Co-authored-by: Michael McMahon <70538289+Michael-Mc-Mahon at users.noreply.github.com>
 - Update src/jdk.httpserver/share/classes/com/sun/net/httpserver/HttpExchange.java
   
   Co-authored-by: Michael McMahon <70538289+Michael-Mc-Mahon at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18955/files
  - new: https://git.openjdk.org/jdk/pull/18955/files/a0a364dd..6bb8d511

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18955&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18955&range=00-01

  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 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