RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable [v4]

[Julia Boes]

> The fix makes the map in sun.net.httpserver.UnmodifiableHeaders unmodifiable by wrapping it in an unmodifiable view.

Julia Boes has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Account for null values and change type in constructor
 - Merge branch 'master' into 8263506
 - Merge branch 'master' into 8263506
 - move constructor call in ExchangeImpl and fix indentation
 - wrap List<String> with unmodifiable map and update test
 - remove map wrapping
 - fix imports
 - Merge branch 'master' into 8263506
 - fix UnmodifiableHeaders impl
 - add initial test

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3032/files
  - new: https://git.openjdk.java.net/jdk/pull/3032/files/8981b444..ee8da732

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=02-03

  Stats: 41260 lines in 1991 files changed: 23607 ins; 9211 del; 8442 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3032.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3032/head:pull/3032

PR: https://git.openjdk.java.net/jdk/pull/3032