Automatically encode data to base64 if needed#153

Based on the discussion in #148. Some data should be encoded to base64 before sending Example: ``` def text_and_text_attachment_failure(): """ This function sends a text body and text attachment. This function fails. """ from django.core.mail import EmailMultiAlternatives msg = EmailMultiAlternatives( subject='Subject', body='Body', from_email='[email protected]', to=['[email protected]'] ) msg.attach('hello.txt', 'Hello World', 'text/plain') msg.send(fail_silently=False) ``` Output: ``` 2018-05-27 09:23:24,588 - Postmarker - DEBUG - Request: POST https://api.postmarkapp.com/email/batch, Data: ({'TrackOpens': False, 'Attachments': [{'Name': 'hello.txt', 'Content': 'Hello World', 'ContentType': 'text/plain'}], 'TextBody': 'Body', 'From': '[email protected]', 'Cc': None, 'ReplyTo': None, 'Bcc': None, 'To': '[email protected]', 'Tag': None, 'HtmlBody': None, 'Headers': [], 'Subject': 'Subject'},) 2018-05-27 09:23:26,211 - Postmarker - DEBUG - Response: [{"ErrorCode":300,"Message":"Invalid attachment content - illegal base64 string."}] Traceback (most recent call last): File "minimal.py", line 78, in <module> text_and_text_attachment_failure() File "minimal.py", line 70, in text_and_text_attachment_failure msg.send(fail_silently=False) File "/home/vagrant/postmarker-test/venv-django-1.11/lib/python3.5/site-packages/django/core/mail/message.py", line 348, in send return self.get_connection(fail_silently).send_messages([self]) File "/home/vagrant/postmarker-test/venv-django-1.11/lib/python3.5/site-packages/postmarker/django/backend.py", line 71, in send_messages self.raise_for_response(not_sent) File "/home/vagrant/postmarker-test/venv-django-1.11/lib/python3.5/site-packages/postmarker/django/backend.py", line 89, in raise_for_response raise PostmarkerException(message) postmarker.exceptions.PostmarkerException: [300] Invalid attachment content - illegal base64 string. ``` Django automatically encodes some content - https://github.com/django/django/blob/master/django/core/mail/message.py#L358 So, the following is not encoded: - basetype is `text` - type is `message/rfc822`