RFR: 8252767: URLConnection.setRequestProperty throws IllegalAccessError [v3]

[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?

Jaikiran Pai has refreshed the contents of this pull request, and previous commits have been removed. The incremental
views will show differences compared to the previous content of the PR. The pull request contains one new commit since
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  8252767: URLConnection.setRequestProperty throws IllegalAccessError
  Summary: Throw an IllegalStateException from sun.net.www.URLConnection#setRequestProperty when already connected
  Reviewed-By:

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/26/files
  - new: https://git.openjdk.java.net/jdk/pull/26/files/faf32fcf..a819f50c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=26&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=26&range=01-02

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