Fix sendgrid attachment sending error

[c-w]

Stacktrace:

  File "/app/opwen_email_server/actions.py", line 32, in __call__
    return self._action(*args, **kwargs)
  File "/app/opwen_email_server/actions.py", line 59, in _action
    success = self._send_email(email)
  File "/app/opwen_email_server/services/sendgrid.py", line 43, in __call__
    return self._send_email(email, email_id)
  File "/app/opwen_email_server/services/sendgrid.py", line 49, in _send_email
    status = self._client(request)
  File "/app/opwen_email_server/services/sendgrid.py", line 35, in send_email
    response = client.client.mail.send.post(request_body=email)
  File "/venv/lib/python3.6/site-packages/python_http_client/client.py", line 236, in http_request
    kwargs['request_body']).encode('utf-8')
  File "/usr/local/lib/python3.6/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/local/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/local/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/local/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'bytes' is not JSON serializable