RFR: 8331195: Improve com.sun.net.httpserver.HttpExchange usability [v4]

[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 two additional commits since the last revision:

 - Merge remote-tracking branch 'robaho/HttpExchange-api-change' into HttpExchange-api-change
   
   # Conflicts:
   #	src/jdk.httpserver/share/classes/com/sun/net/httpserver/HttpExchange.java
 - update api changes based on comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18955/files
  - new: https://git.openjdk.org/jdk/pull/18955/files/3af58d0c..a30345a1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18955&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18955&range=02-03

  Stats: 22 lines in 1 file changed: 14 ins; 1 del; 7 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