Fix util.stringify for null objects.

jIO.util.stringify({y: null}) used to error. Now it behaves similarly
to JSON.stringify, returning '{"y":null}'.

src/jio.js

@@ -132,6 +132,9 @@
     if (obj === undefined) {
       return undefined;
     }
+    if (obj === null) {
+      return 'null';
+    }
     if (obj.constructor === Object) {
       key_list = Object.keys(obj).sort();
       result_list = [];

test/jio/util.js

@@ -24,6 +24,8 @@
     equal(str(Object.create(null, { x: { value: 'x', enumerable: false },
                                     y: { value: 'y', enumerable: true } })),
           '{"y":"y"}');
+    equal(str({y: "y", testnull: null}),
+          '{"testnull":null,"y":"y"}');
 
   });