def on_air?(arg)
      date = to_date(arg)

      return false unless respond_to?(:started_date)

      if respond_to?(:ended_date)
        # ended title
        (started_date..ended_date).cover?(date)
      else
        # on air title
        started_date <= date
      end
    end
def on_air?(arg)
      date = to_date(arg)

      return false unless respond_to?(:started_date)

      if respond_to?(:ended_date)
        # ended title
        (started_date..ended_date).cover?(date)
      else
        # on air title
        started_date <= date
      end
    end
@param [Rubicure::Series,Rubicure::Girl] other

 @return [Boolean] other is same Rubicure::Series or Rubicure::Series include Rubicure::Girl
 Whether series is on air

 @param [Time,Date,String] arg Time, Date or date like String (ex. "2013-12-16")

 @return [Boolean]