Return new instances of PYSON object from PYSON shortcut methods

Closes #12985
(grafted from af58504c1e6ad2b17cac5b6c41454d7eda28c5c7)

--HG--
branch : 6.0

sao/src/pyson.js

@@ -36,13 +36,13 @@
             if (d === undefined) {
                 d = '';
             }
-            return Sao.PYSON.Get(this, k, d);
+            return new Sao.PYSON.Get(this, k, d);
         },
         in_: function(obj) {
-            return Sao.PYSON.In(this, obj);
+            return new Sao.PYSON.In(this, obj);
         },
         contains: function(k) {
-            return Sao.PYSON.In(k, this);
+            return new Sao.PYSON.In(k, this);
         },
         toString: function() {
             var klass = this.pyson().__class__;

sao/tests/sao.js

@@ -1263,6 +1263,9 @@
         QUnit.strictEqual(
             eval_pyson('Or(True, False).toString()'),
             "Or(true, false)", "eval_pyson('Or(True, False)').toString()");
+        QUnit.strictEqual(
+            eval_pyson('Eval("foo", {}).get("bar").toString()'),
+            'Get(Eval("foo", {}), "bar", "")');
     });
 
     QUnit.test('DomainParser.group_operator', function() {