RFR: 8289260: BigDecimal movePointLeft() and movePointRight() do not follow their API spec

[Raffaello Giulietti]

`BigDecimal.morePoint[Left|Right]()` should return the target `this` when the argument is 0 _and_ the scale is non-negative.

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Commit messages:
 - 8289260: BigDecimal movePointLeft() and movePointRight() do not follow their API spec
 - 8289260: BigDecimal movePointLeft() and movePointRight() do not follow their API spec

Changes: https://git.openjdk.org/jdk/pull/9307/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9307&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289260
  Stats: 76 lines in 2 files changed: 74 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9307.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9307/head:pull/9307

PR: https://git.openjdk.org/jdk/pull/9307