: disposition( default = nil ) -> String
: disposition=( pos )
    pos: String

    Content-Disposition μ (ʸ)֤ͤϾ˾ʸ줵ޤ
    name бͤإåΤΤ¸ߤʤ default 
    ֤ޤ
      --
      # example
      mail['Content-Disposition'] = 'attachement; filename="test.rb"'
      p mail.disposition   # "attachment"
      --

