Ensure rec_name is a char, text type

Model.default_get with 'default_rec_name' context relies on the fact that
rec_name is a string base field.

issue6630
review36501002

product.py

@@ -22,7 +22,6 @@ class ProductCostPrice:
 class ProductCostHistory(ModelSQL, ModelView):
     'History of Product Cost'
     __name__ = 'product.product.cost_history'
-    _rec_name = 'date'
     product = fields.Many2One('product.product', "Product")
     date = fields.DateTime('Date')
     cost_price = fields.Numeric('Cost Price')
@@ -50,6 +49,9 @@ class ProductCostHistory(ModelSQL, ModelView):
                 Coalesce(history.write_date, history.create_date),
                 history.product, history.cost_price))
 
+    def get_rec_name(self, name):
+        return str(self.date)
+
 
 class OpenProductCostHistory(Wizard):
     'Open Product Cost History'