    # File lib/action_mailer/vendor/tmail-1.2.3/tmail/interface.rb, line 699
        def mime_version=( m, opt = nil )
          if opt
            if h = @header['mime-version']
              h.major = m
              h.minor = opt
            else
              store 'Mime-Version', "#{m}.#{opt}"
            end
          else
            store 'Mime-Version', m
          end
          m
        end