def trim! desired_row_length=nil
      max_length = self.collect{|a| a.trim.length }.max
      self_count = self.count-1
      self.count.times do |index|
        index = self_count - index
        if self[index].trim.empty?
          self.delete_at(index)
        else
          break
        end
      end
      self.each{|a| a.trim!(max_length)}
      self
    end
def trim! desired_row_length=nil
      max_length = self.collect{|a| a.trim.length }.max
      self_count = self.count-1
      self.count.times do |index|
        index = self_count - index
        if self[index].trim.empty?
          self.delete_at(index)
        else
          break
        end
      end
      self.each{|a| a.trim!(max_length)}
      self
    end
remove all the trailing empty-rows (returning a trimmed self)

 @param [Integer] desired_row_length of the new row
 @return [Workbook::Row] self