Update AssertOperator and RefuteOperator to allow :[]

[flavorjones]

Update the AssertOperator and RefuteOperator cops to allow :[]

The rubocop style guide prefers assert and refute for testing booleans. If I have an array of booleans, then I want to be able to use this style:

  def test_my_array
    assert(array_of_bools[0])
    refute(array_of_bools[1])
  end

This PR is suggested by an actual test in the Nokogiri suite which collects boolean values from Nokogiri::Reader as it passes through the document, and then checks the values after completing the parsing pass. That test could be rewritten, but I don't personally think it would be made more readable by that rewrite.

Also see #260 (comment) which raised the question of whether :[] was intentionally a violation.

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[koic]

This seems like a reasonable change. Thank you!