--- default
--- default([key])                ruby 1.7 feature

    ϥåΥǥե֤ͤޤ

    ruby 1.7 feature:
    2 ܤηϥϥå夬ǥեͤȤƥ֥åľ
    (Hash.new)˻Ǥޤself   
    key ˥֥å¹ԤƤη̤֤ޤ
    key ξάͤ nil Ǥ

