Fix autocorrection bug when ::Hash is used on Rails/IndexBy and Rails/IndexWith

[r7kamura]

The following 2 cops currently support both Hash[...] and ::Hash[...] , but the autocorrection for ::Hash[...] yields incorrect result. This is because it assumes only Hash[ and removes the first 5 characters from source code.

• Rails/IndexBy
• Rails/IndexWith

This problem was originally found at the following Pull Request:

• Fix false-positives when constant is used with receiver on Rails/DurationArithmetic, Rails/IndexBy, Rails/IndexWIth, and Rails/RequireDependency #866 (comment)

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Great! Thanks!