Update specs to use the expect_correction helper

[Darhazer]

While working on a fix for #712 I've noticed that the specs of Rails/Delegate use the old-fashioned auto-correction validation. I've updated everywhere that it doesn't make sense to use autocorrect_source (some cops still use it as they have additional specs on the correction and there is little gain in concerting those; and others because of the shared_examples usage)

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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]

Thanks!