tests: change default task serializer
tests: change default task serializer

* Celery 4 changes default task serializer from `pickle`
  to `json` and `test_send_message_with_attachments` which
  has a binary file as attachment fails as the attachment is
  not JSON serializable (closes #47).

inveniosoftware/invenio-mail,inveniosoftware/invenio-mail,inveniosoftware/invenio-mail