[FIX] web,pad: ensure pad content is displayed after discard

Up to this revision, clicking on 'edit' in a record with an existing pad
and discarding the changes straight away caused the pad content not to
be displayed.

The root cause of this issue is twofold:

- a hack tricks the model into thinking the pad URL has changed by
  replacing it with an object yielding the URL when JSONified;
- the pad never affects the 'dirtiness' of a record and BasicModel only
  actually discards changes when the record is dirty.

When a user clicks on discard, the model does not actually discard the
changes, thereby leaving the web client into thinking the pad URL is the
object mentioned above.

We fix this issue by requiring that the model always discard changes
when clicking 'Discard', regardless of whether the record is dirty.
In fact, the lines we changed were no longer correct since the
introduction of 'doNotSetDirty' (see
https://github.com/odoo/odoo/commit/72ed3a1), as a record could be
changed without being dirty. This commit thus also ensures that such
records are also properly discarded.

addons/pad/static/tests/pad_tests.js

@@ -230,4 +230,37 @@ QUnit.module('pad widget', {
         delete FieldPad.prototype.isPadConfigured;
     });
 
+    QUnit.test('record should be discarded properly even if only pad has changed', function (assert) {
+        assert.expect(1);
+
+        var form = createView({
+            View: FormView,
+            model: 'task',
+            data: this.data,
+            arch:'<form>' +
+                    '<sheet>' +
+                        '<group>' +
+                            '<field name="description" widget="pad"/>' +
+                        '</group>' +
+                    '</sheet>' +
+                '</form>',
+            res_id: 2,
+            mockRPC: function (route, args) {
+                if (!args.method) {
+                    return $.when(true);
+                }
+                return this._super.apply(this, arguments);
+            },
+            session: {
+                userName: "batman",
+            },
+        });
+        form.$buttons.find('.o_form_button_edit').click();
+        form.$buttons.find('.o_form_button_cancel').click();
+        assert.strictEqual(form.$('.oe_pad_readonly').text(), this.data.task.pad_get_content(),
+            "pad content should not have changed");
+        form.destroy();
+        delete FieldPad.prototype.isPadConfigured;
+    });
+
 });

addons/web/static/src/js/views/basic/basic_controller.js

@@ -329,9 +329,7 @@ var BasicController = AbstractController.extend(FieldManagerMixin, {
                 if (options && options.readonlyIfRealDiscard && !needDiscard) {
                     return;
                 }
-                if (needDiscard) { // Just some optimization
-                    self.model.discardChanges(recordID);
-                }
+                self.model.discardChanges(recordID);
                 if (self.model.isNew(recordID)) {
                     self._abandonRecord(recordID);
                     return;