jio.util.stringify: Add support for undefined values

`jIO.util.stringify` was raising if the value of an object was undefined, this changes fix it and provides the same behaviour as the standard `JSON.stringify`

/reviewed-on nexedi/jio!41

src/jio.js

@@ -124,6 +124,9 @@
       i,
       value,
       result_list;
+    if (obj === undefined) {
+      return undefined;
+    }
     if (obj.constructor === Object) {
       key_list = Object.keys(obj).sort();
       result_list = [];

test/jio/util.js

@@ -20,6 +20,7 @@
           '"2006-01-02T15:04:05.000Z"');
     equal(str({ x: 5, y: 6, z: 7 }), '{"x":5,"y":6,"z":7}');
     equal(str({ z: 7, y: 6, x: 5 }), '{"x":5,"y":6,"z":7}');
+    equal(str({ z: "", y: undefined, x: 5 }), '{"x":5,"z":""}');
     equal(str(Object.create(null, { x: { value: 'x', enumerable: false },
                                     y: { value: 'y', enumerable: true } })),
           '{"y":"y"}');