[FIX] various: adapt code to new datetime fields

Some code was still using the date and datetime fields as if they were
strings, but they are now dates and datetimes

addons/auth_signup/models/res_partner.py

@@ -18,8 +18,7 @@ def random_token():
     return ''.join(random.SystemRandom().choice(chars) for _ in range(20))
 
 def now(**kwargs):
-    dt = datetime.now() + timedelta(**kwargs)
-    return fields.Datetime.to_string(dt)
+    return datetime.now() + timedelta(**kwargs)
 
 
 class ResPartner(models.Model):

addons/purchase_stock/models/stock_rule.py

@@ -88,7 +88,7 @@ class StockRule(models.Model):
         seller = product_id._select_seller(
             partner_id=partner,
             quantity=product_qty,
-            date=fields.Date.to_string(schedule_date),
+            date=schedule_date and schedule_date.date(),
             uom_id=product_uom)
 
         return schedule_date - relativedelta(days=int(seller.delay))

addons/sale_purchase/models/sale_order.py

@@ -220,7 +220,7 @@ class SaleOrderLine(models.Model):
         supplierinfo = self.product_id._select_seller(
             partner_id=purchase_order.partner_id,
             quantity=purchase_qty_uom,
-            date=purchase_order.date_order, # and purchase_order.date_order[:10],
+            date=purchase_order.date_order and purchase_order.date_order.date(), # and purchase_order.date_order[:10],
             uom_id=self.product_id.uom_po_id
         )
         fpos = purchase_order.fiscal_position_id

addons/stock/models/stock_warehouse.py

@@ -924,7 +924,7 @@ class Orderpoint(models.Model):
             # These days will be substracted when creating the PO
             days += self.product_id._select_seller(
                 quantity=product_qty,
-                date=fields.Date.to_string(start_date),
+                date=start_date,
                 uom_id=self.product_uom).delay or 0.0
         date_planned = start_date + relativedelta.relativedelta(days=days)
         return date_planned.strftime(DEFAULT_SERVER_DATETIME_FORMAT)