Send SMTP mail using quoted-printable encoding in all cases

Summary: See D10278. This works around a bug (?) in SendGrid, see http://stackoverflow.com/questions/6276181/extra-newlines-in-plain-text-emails-sent-via-sendgrid

Test Plan: @nickz confirmed this resolved his issue. See also D10278.

Reviewers: nickz, btrahan, chad

Reviewed By: chad

Subscribers: nickz, epriestley

Differential Revision: https://secure.phabricator.com/D10303