Only update ec_sales_list_code when it's diferent from template value

issue7992
review49711002

account.py

@@ -16,7 +16,8 @@ class TaxTemplate(metaclass=PoolMeta):
 
     def _get_tax_value(self, tax=None):
         value = super(TaxTemplate, self)._get_tax_value(tax=tax)
-        value['ec_sales_list_code'] = self.ec_sales_list_code
+        if not tax or tax.ec_sales_list_code != self.ec_sales_list_code:
+            value['ec_sales_list_code'] = self.ec_sales_list_code
         return value