[FIX] point_of_sale: correct call to default value

The default value should be called with a lambda otherwise the payment_date is set to the date the server was started (opw 606715)

addons/point_of_sale/wizard/pos_payment.py

@@ -126,7 +126,7 @@ class pos_make_payment(osv.osv_memory):
     }
     _defaults = {
         'journal_id' : _default_journal,
-        'payment_date': time.strftime('%Y-%m-%d %H:%M:%S'),
+        'payment_date': lambda *a: time.strftime('%Y-%m-%d %H:%M:%S'),
         'amount': _default_amount,
     }