# File lib/action_mailer/vendor/tmail-1.2.3/tmail/interface.rb, line 861
    def charset=( str )
      if str
        if h = @header[ 'content-type' ]
          h['charset'] = str
        else
          store 'Content-Type', "text/plain; charset=#{str}"
        end
      end
      str
    end