Make Lint/DuplicateMethods aware of attr_* methods

[pocke]

Lint/DuplicateMethods cop does not offenses to the following code.

class User
  attr_reader :name

  # This definition is unnecessary, but RuboCop does not detect it.
  def name
    @name
  end
end

This change fixes the problem.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• 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.
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• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[olivierlacan]

This cop causes false positives on reader method overrides that don't simply return the associated instance variable.

[SwagDevOps]

Nope, even method does not simply return, it is a duplicate.