[FIX] sale: Server Error when trying to process card

Due to this commit: b367625248
The function "_get_payment_type" was not defined in module "sale_payment" but defined in "website_quote"
The bridge module for "website_quote" and "sale_payment" is "sale" so the first definition of "_get_payment_type"
must be in "sale".

opw:1881990

addons/sale/models/sale.py

@@ -112,6 +112,11 @@ class SaleOrder(models.Model):
         for order in self:
             order.order_line._compute_tax_id()
 
+    @api.multi
+    def _get_payment_type(self):
+        self.ensure_one()
+        return 'form'
+
     name = fields.Char(string='Order Reference', required=True, copy=False, readonly=True, states={'draft': [('readonly', False)]}, index=True, default=lambda self: _('New'))
     origin = fields.Char(string='Source Document', help="Reference of the document that generated this sales order request.")
     client_order_ref = fields.Char(string='Customer Reference', copy=False)