[FIX] mass_mailing: replace customer by customer_rank
[FIX] mass_mailing: replace customer by customer_rank

Since odoo/odoo@8b7f42ab and odoo/odoo@8766f388 fields customer
and supplier on `res.partner` have been replaced with
`customer_rank` and `supplier_rank`

closes odoo/odoo#36067

Signed-off-by: Thibault Delavallee (tde) <2d325771bb6e8870d87f0f3f7b45589a8338e550@openerp.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo