Return default value when evaluating undefined value

issue11089
review374511002
(grafted from 21cc9ddb278906609bbdee15aaa17d03422476e6)

--HG--
branch : 6.0

src/pyson.js

@@ -179,7 +179,7 @@
                 'd': value.d,
             }, context[value.v.substring(0, idx)] || {});
         }
-        if (value.v in context) {
+        if ((value.v in context) && (context[value.v] !== undefined)) {
             return context[value.v];
         } else {
             return value.d;

tests/sao.js

@@ -102,6 +102,9 @@
             "decode(Eval('test', 0))");
         QUnit.strictEqual(new Sao.PYSON.Decoder().decode(eval_), 0,
             "decode(Eval('test', 0))");
+        QUnit.strictEqual(
+            new Sao.PYSON.Decoder({test: undefined}).decode(eval_), 0,
+            "decode(Eval('test', 0))");
     });
 
     QUnit.test('PYSON Not', function() {