Use on_change on default_uom instead of sale_uom

Changing the value of sale_uom when it is changed, prevents the user to select
an other value than the default one because when the user remove the current
value it is set back to the default one.

issue4237
review11561002

product.py

@@ -52,15 +52,20 @@ class Template:
         return 0
 
     @fields.depends('default_uom', 'sale_uom', 'salable')
-    def on_change_with_sale_uom(self):
+    def on_change_default_uom(self):
+        try:
+            changes = super(Template, self).on_change_default_uom()
+        except AttributeError:
+            changes = {}
         if self.default_uom:
             if self.sale_uom:
                 if self.default_uom.category == self.sale_uom.category:
-                    return self.sale_uom.id
+                    changes['sale_uom'] = self.sale_uom.id
                 else:
-                    return self.default_uom.id
+                    changes['sale_uom'] = self.default_uom.id
             else:
-                return self.default_uom.id
+                changes['sale_uom'] = self.default_uom.id
+        return changes
 
 
 class Product: