[Intl] Updated stubs to reflect ICU 51.2
merged branch bschussek/icu51 (PR #8680)

This PR was merged into the 2.3 branch.

Discussion
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[Intl] Updated stubs to reflect ICU 51.2

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In addition to the upgrade of the ICU component to ICU 51.2, this PR upgrades the stub classes to behave like their C counterparts do with ICU 51.2.

Commits
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8fa0453 [Intl] Updated stubs to reflect ICU 51.2