Mark Rails/UniqBeforePluck cop as unsafe auto-correction

[koic]

Follow #320 and rubocop/rubocop#8529.

This PR changes the mark of Rails/UniqBeforePluck from Safe: false to SafeAutoCorrect: false.

False positives to detection are not a concern. Only worry about incompatibilities with auto-correction.

Below is an excerpt from the official document.

• Safe (true/false) - indicates whether the cop can yield false positives (by design) or not.

• SafeAutoCorrect (true/false) - indicates whether the auto-correct a cop does is safe (equivalent) by design. If a cop is unsafe its auto-correct automatically becomes unsafe as well.

https://docs.rubocop.org/rubocop/0.89/usage/auto_correct.html#safe-auto-correct

Cc @kunitoo

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.