Spurious = displayed on quoted-printable emails from ActionMailer

This happens because of https://github.com/mikel/mail/issues/612 . 

When we send quoted-printable from ActionMailer, we always see an extra '=' in the display in MailCatcher. It's not clear where to really fix the underlying problem, so I've worked around it here. I have *not* tested this comprehensively (I'd want to check multipart more aggressively).

PR includes example file and workaround.