[FIX] web: correctly apply onchange on boolean field

When an onchange sets a boolean field to 'true', if this boolean was set to 'false'
previously, the old value was used instead of 'false'.

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

@@ -1235,7 +1235,7 @@ var BasicModel = AbstractModel.extend({
                 record._rawChanges[name] = val;
                 return;
             }
-            var oldValue = record._changes[name] || record.data[name];
+            var oldValue = name in record._changes ? record._changes[name] : record.data[name];
             var id;
             if (field.type === 'many2one') {
                 id = false;

addons/web/static/tests/views/basic_model_tests.js

@@ -2123,4 +2123,47 @@ QUnit.module('Views', {
         model.destroy();
     });
 
+    QUnit.test('onchange on a boolean field', function (assert) {
+        assert.expect(2);
+
+        var newFields = {
+            foobool: {
+                type: 'boolean',
+                string: 'foobool',
+            },
+            foobool2: {
+                type: 'boolean',
+                string: 'foobool2',
+            },
+        };
+        _.extend(this.data.partner.fields, newFields);
+
+        this.data.partner.onchanges.foobool = function (obj) {
+            if (obj.foobool) {
+                obj.foobool2 = true;
+            }
+        };
+
+        this.data.partner.records[0].foobool = false;
+        this.data.partner.records[0].foobool2 = true;
+
+        this.params.res_id = 1;
+        this.params.fieldNames = ['foobool', 'foobool2'];
+        this.params.fields = this.data.partner.fields;
+        var model = createModel({
+            Model: BasicModel,
+            data: this.data,
+        });
+
+        model.load(this.params).then(function (resultID) {
+            var record = model.get(resultID);
+            model.notifyChanges(resultID, {foobool2: false});
+            record = model.get(resultID);
+            assert.strictEqual(record.data.foobool2, false, "foobool2 field should be false");
+            model.notifyChanges(resultID, {foobool: true});
+            record = model.get(resultID);
+            assert.strictEqual(record.data.foobool2, true, "foobool2 field should be true");
+        });
+        model.destroy();
+    });
 });});