RFR: 8247918: Clarify Reader.skip behavior for end of stream

[Brian Burkhalter]

Please review this clarification of the specification of the method `skip(long)` in `java.io.Reader` and its subclasses. Specifically, the behavior of the method is made clear for the case when the `Reader` is already at the end of its stream when the method is invoked. A corresponding CSR will be filed. Also, the change includes an update to an existing test in order to verify that the specification change reflects actual behavior.

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Commit messages:
 - 8247918: Clarify Reader.skip behavior for end of stream

Changes: https://git.openjdk.java.net/jdk/pull/2274/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2274&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247918
  Stats: 132 lines in 8 files changed: 62 ins; 38 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2274.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2274/head:pull/2274

PR: https://git.openjdk.java.net/jdk/pull/2274