RFR: 8269692: sun.net.httpserver.ServerImpl::createContext should throw IAE

[Julia Boes]

`com.sun.net.httpserver.HttpServer::createContext` specifies `IllegalArgumentException` to be thrown if a context already exists for the path provided. The implementation class`sun.net.httpserver.ServerImpl` does not comply with this, which is fixed by this change.

Testing: Tier 1-3 all clear.

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Commit messages:
 - fix trailing whitespace
 - use HttpServer, not HttpsServer in test
 - initial commit

Changes: https://git.openjdk.java.net/jdk/pull/4665/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4665&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269692
  Stats: 88 lines in 2 files changed: 84 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4665.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4665/head:pull/4665

PR: https://git.openjdk.java.net/jdk/pull/4665