RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable

[Julia Boes]

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

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Commit messages:
 - fix imports
 - Merge branch 'master' into 8263506
 - fix UnmodifiableHeaders impl
 - add initial test

Changes: https://git.openjdk.java.net/jdk/pull/3032/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263506
  Stats: 146 lines in 2 files changed: 136 ins; 1 del; 9 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