Fix failing of AM - https://travis-ci.org/rails/rails/jobs/7286782

- Failing test was passing "base64" as content_transfer_encoding.
- When a content_transfer_encoding is given explicitly, mail expects
  data to be sent also with the attachment.
- Acc. to https://github.com/mikel/mail/blob/master/lib/mail/attachments_list.rb#L90
  default transfer_encoding is either "binary" or "7bit".
- As we are testing the expected value with binary decoded data,
  passing content_transfer_encoding as "binary" fixes the problem.