[FIX] mail: correctly parse body as html for pure-html email

Some external tools send email as pure html (no multipart) and when
parsing such email we ends up having the raw HTML as body (text)

This commit ensure we correctly parse and sanitize the body as HTML
for such emails.

closes odoo/odoo#130489

Task-id: 3451889
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>