RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable [v3]

[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 incrementally with one additional commit since the last revision:

  move constructor call in ExchangeImpl and fix indentation

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3032/files
  - new: https://git.openjdk.java.net/jdk/pull/3032/files/fcdfb182..8981b444

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=01-02

  Stats: 52 lines in 2 files changed: 0 ins; 8 del; 44 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