instance method with_multipart_encoding

# File actionmailer/lib/action_mailer/vendor/tmail-1.2.7/tmail/mail.rb, line 190
    def with_multipart_encoding( strategy )
      if parts().empty?    # DO NOT USE @parts
        yield

      else
        bound = ::TMail.new_boundary
        if @header.key? 'content-type'
          @header['content-type'].params['boundary'] = bound
        else
          store 'Content-Type', %<multipart/mixed; boundary="#{bound}">
        end

        yield

        parts().each do |tm|
          strategy.puts
          strategy.puts '--' + bound
          tm.accept strategy
        end
        strategy.puts
        strategy.puts '--' + bound + '--'
        strategy.write epilogue()
      end
    end