: type_param( name, default = nil ) -> String
    name: String

    Content-Type  name ѥ᡼֤ͤޤ
    name бͤإåΤΤ¸ߤʤ default 
    ֤ޤ
      --
      # example
      mail['Content-Type'] = 'text/plain; charset=iso-2022-jp'
      p mail.type_param('charset')   # "iso-2022-jp"
      --

