RFR: 8252767: URLConnection.setRequestProperty throws IllegalAccessError

[Jaikiran Pai]

Can I please get a review and a sponsor for a fix for the issue reported at
https://bugs.openjdk.java.net/browse/JDK-8252767?

As noted in that issue, the `sun.net.www.URLConnection#setRequestProperty` is throwing a `IllegalAccessError` instead
of a `IllegalStateException`. The commit here fixes that and includes a test which reproduces the issue and verifies
the fix.

Would a CSR be needed for this change?

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Commit messages:
 - 8252767: URLConnection.setRequestProperty throws IllegalAccessError

Changes: https://git.openjdk.java.net/jdk/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=26&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252767
  Stats: 64 lines in 2 files changed: 63 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/26/head:pull/26

PR: https://git.openjdk.java.net/jdk/pull/26