[FIX] web: image widget is dependent on its record's last_update

As a way to optimize loading, images are not necessarily fetched in db.
They have, in their url a "unique" parameter, which is the last_update date on **the record** and controls on the python-side whether it should get the image from a cache or from the db.

Before this commit, this __last_update field wasn't present in the view, so it wasn't fetched, and writes on a model's image worked but did not refresh.
The image displayed was the old one.

After this commit, when the image field widget is present, we force the loading of the __last_update field of the record.
Upon update, the image displayed is the new one.

OPW 777552

closes #20457

addons/web/static/tests/fields/basic_fields_tests.js

@@ -1427,8 +1427,10 @@ QUnit.module('basic_fields', {
     QUnit.module('FieldImage');
 
     QUnit.test('image fields are correctly rendered', function (assert) {
-        assert.expect(5);
+        assert.expect(6);
 
+        this.data.partner.fields.__last_update = {string: "Last updated", type: "datetime"};
+        this.data.partner.records[0].__last_update = '2017-02-08 10:00:00';
         this.data.partner.records[0].document = 'myimage';
 
         var form = createView({
@@ -1439,7 +1441,10 @@ QUnit.module('basic_fields', {
                     '<field name="document" widget="image" options="{\'size\': [90, 90]}"/> ' +
                 '</form>',
             res_id: 1,
-            mockRPC: function (route) {
+            mockRPC: function (route, args) {
+                if (route === '/web/dataset/call_kw/partner/read') {
+                    assert.deepEqual(args.args[1], ['document', '__last_update', 'display_name'], "The fields document, display_name and __last_update should be present when reading an image");
+                }
                 if (route === 'data:image/png;base64,myimage') {
                     assert.ok(true, "should called the correct route");
                     return $.when('wow');