RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable [v2]

[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 two additional commits since the last revision:

 - wrap List<String> with unmodifiable map and update test
 - remove map wrapping

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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/93b88fce..fcdfb182

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=00-01

  Stats: 43 lines in 2 files changed: 11 ins; 8 del; 24 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