def set_length(len , fill_char)
      return if len <= 0
      old = char_length
      return if old >= len
      self.char_length = len
      check_length
      fill_from_with( old + 1 , fill_char )
    end
def set_length(len , fill_char)
      return if len <= 0
      old = char_length
      return if old >= len
      self.char_length = len
      check_length
      fill_from_with( old + 1 , fill_char )
    end
pad the string with the given character to the given length