[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#132020

Task-id: 3451889
X-original-commit: b078f717
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Signed-off-by: Xavier Alt (xal) <xal@odoo.com>