string#hyphenize: run test only if __FILE__ == $PROGRAM_NAME
ruby/integer#factorial: prefer double-quotes
ruby/string#undent: prefer double-quotes
ruby/string#percent_encode: prefer double-quotes
ruby/string#hyphenize: prefer double-quotes
ruby/string#percent_encode: prefer double-quotes
ruby/integer#factorial: add underscores to large numeric literals
ruby/integer#factorial: returns nil for negative number, and 1 for 0
ruby/string#palindrome: check whether a string is palindrome or not