Mark Rails/WhereExists as unsafe auto-correction

[koic]

Summary

This cop is unsafe because the behavior may change on the following case:

Author.includes(:articles).where(articles: {id: id}).exists?
#=> Perform `eager_load` behaviour (`LEFT JOIN` query) and get result.

Author.includes(:articles).exists?(articles: {id: id})
#=> Perform `preload` behaviour and `ActiveRecord::StatementInvalid` error occurs.

Additional Information

And this cop may be considered disabled by default as there is no consensus on the default style.

• Add new Rails/WhereExists cop #286
• [Fix #341] Add EnforcedStyle to Rails/WhereExists to allow where(...).exists? to be the enforced style #342

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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.
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.