mail: set content-transfer-encoding to 8bit
This is a workaround to a lettre bug, which sets Content-Type in emails to "text/plain; charset=utf-8", but doesn't set the transfer encoding accordingly. This rarely causes errors, but we should strive to be correct.
See related issue: #144 (closed)
Upstream issue: https://github.com/lettre/lettre/issues/400
I've been slightly dissatisfied with lettre's stability and attention to correctness. I'll add some unit tests to nail down its behavior a bit better.