def valid? number
      raise_on_type_mismatch number
      num, check = split_isbn_number(number)
      convert_number_to_digits(num).length == 9 && self.of(num) == check
    end
def valid? number
      raise_on_type_mismatch number
      num, check = split_isbn_number(number)
      convert_number_to_digits(num).length == 9 && self.of(num) == check
    end
ISBN-10 validation of provided number

 @example
   Sjekksum::ISBN10.valid?("1477430253") #=> true
   Sjekksum::ISBN10.valid?("193435600X") #=> true

 @param  number [Integer, String] number with included checksum

 @return [Boolean]