--- to_s
--- to_s(base)       ruby 1.7 feature

     10 ʸɽѴޤ

    ruby 1.7 feature: ꤹСȤʸɽ
    ѴޤȤ 2  36 ʳꤷ㳰 
    ArgumentError ȯޤ

        p 10.to_s(2)    # => "1010"
        p 10.to_s(8)    # => "12"
        p 10.to_s(16)   # => "a"
        p 35.to_s(36)   # => "z"

