RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable [v6]

[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:

  assign unmodHeaders instead of original headers

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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/4f2247fc..6fce0d2d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3032&range=04-05

  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 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