I noticed that when running autocorrect for Style/AccessModifierDeclarations, the method above the group declaration was being deleted. So this:

class MyThing
  def other_method; end

  def a_method_that_is_public; end

  private

  def my_other_private_method; end
end

After going through autocorrect would become:

class MyThing
  def other_method; end

  private

  private def my_other_private_method; end
end

This PR adds a spec demonstrating the behavior with as small a code sample as I could get down to, and then fixes the issue - when trying to change the code it was always using index 1, which wasn't correct if the AST had the entire class body for what needed to be processed.

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.