[FIX] web: update form view image after reload

6baf611d updated the image when showing the view form the first time,
but when reloading (e.g after a write) there was a difference which
would not cause the update.

second half of:
opw-654279

(follow-up of a8ae944)

addons/web/static/src/js/views/form_view.js

@@ -1010,6 +1010,7 @@ var FormView = View.extend(common.FieldManagerMixin, {
             } else {
                 var fields = _.keys(self.fields_view.fields);
                 fields.push('display_name');
+                fields.push('__last_update');
                 return self.dataset.read_index(fields,
                     {
                         context: { 'bin_size': true },