workaround #771: unescape HTML after rendering plain text email
Created by: das-g
connected to #771 (not a final solution, just a hacky workaround)
The added test is the same as in WIP commit 8dda7f54cbc76e17f529b40675ada0a147b2d851 of the (hopefully) final solution that'll become available with Djagno 2.0