Avoid repeat Content-Transfer-Encoding

[destanyol]

We noticed that the shared inbox e-mail client we are using has trouble with some repeated headers of the attachment.

In particular it has trouble with the "Content-Transfer-Encoding" header. Qreu sends it two times like this:

--===============6391006585805361748==
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: application/pdf; charset="utf-8"
Content-Disposition: attachment; filename="file.pdf"
Content-Transfer-Encoding: base64

JVBERi0xLjQKMSAwIG9iago8P...

By default when you create the attachment as a MIMEApplication the Content-Transfer-Encoding header is added by default and also by default is base64, so the Content-Transfer-Encoding doesn't have to be added manually afterwards.

But, because "Explicit is better than implicit" in this Pull Request I have hardcoded the _encoder to base64 so the change is more clear for the reviewer.

The result with the new changes are:

--===============5362653904239654853==
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: application/pdf; charset="utf-8"
Content-Disposition: attachment; filename="file.pdf"

JVBERi0xLjQKMSAwIG9iago8P...

[destanyol]

Closed, new PR in #65