Work around a splat assignment bug in Rubinius

This splat assignment fails on Rubinius 2.2.9 due to its bug:

  b.freeze
  *_conditions, body = *b

  RuntimeError:
    can't modify frozen instance of Array
  # kernel/common/array.rb:988:in `pop'
  # ./lib/rubocop/cop/style/indentation_width.rb:99:in `on_case'

It seems to be fixed on the current master (0a92c3c).