--- Net::FTP#set_socket(sock, get_greeting = true)

    ̿ TCPSocket ֥ sock ꤷޤ
    άǽ2 get_greeting ˵Ϳȡ
    set_socket ̿ꤹ sock Ф
    Ԥʤޤ󡣤ΤȤͤȤ nil ֤ޤ
    get_greeting (ǥե)ʤС̿ sock 
    ꤷȡ̾FTPФ³äƤåɤ߹ߡ
    ΥåͤȤ֤ޤ

