--- function#decode_b(str)
      RFC:2047 Ƥ encoded-word 
      ޤʸstrǥɤ롣

      encoded-word ϡ
         "=?" + charset + "?" + encoding + "?" + encoded-text + "?="
      Ȥʸǡ᡼إå˻Ѥ롣

      δؿ charset Ȥ "iso-2022-jp"  "shift_jis" 
      encoding Ȥ "B"-encoding (Base64) 򥵥ݡȤƤ롣

      : Х
        ºݤϡ Kconv.toeuc ƤǤ뤿 NKF⥸塼
        ˥ǥɤԤäƤޤ(NKF [NKF]򻲾ȤΤ)
        äơδؿϤޤ̣ʤʤäƤޤäƤ롣
        (ΤKconvμNKFѤʤäᤳΤ褦ʤȤˤʤä
        ΤȻפ)

