Have goog.json.serialize return 'null' for functions. This conforms with

JSON.stringify, and allows these serializations to be consumed by other standard server/client-side parsers.

Fixes google#491
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=99202470

Author: joeltine

closure/goog/json/json.js

@@ -238,8 +238,7 @@ goog.json.Serializer.prototype.serializeInternal = function(object, sb) {
       sb.push(object);
       break;
     case 'function':
-      // Skip functions.
-      // TODO(user) Should we return something here?
+      sb.push('null');
       break;
     default:
       throw Error('Unknown type: ' + typeof object);
@@ -347,7 +346,6 @@ goog.json.Serializer.prototype.serializeObject_ = function(obj, sb) {
     if (Object.prototype.hasOwnProperty.call(obj, key)) {
       var value = obj[key];
       // Skip functions.
-      // TODO(ptucker) Should we return something for function properties?
       if (typeof value != 'function') {
         sb.push(sep);
         this.serializeString_(key, sb);

closure/goog/json/json_test.js

@@ -105,10 +105,15 @@ function testArraySerialize() {
   assertSerialize('[1,2]', [1, 2]);
   assertSerialize('[1,2,3]', [1, 2, 3]);
   assertSerialize('[[]]', [[]]);
+  assertSerialize('[null,null]', [function() {}, function() {}]);
 
   assertNotEquals('{length:0}', goog.json.serialize({length: 0}), '[]');
 }
 
+function testFunctionSerialize() {
+  assertSerialize('null', function() {});
+}
+
 function testObjectSerialize_emptyObject() {
   assertSerialize('{}', {});
 }
@@ -134,7 +139,7 @@ function testSerializeSkipFunction() {
     i: 100,
     f: function() { var x = 'x'; }
   };
-  assertSerialize('', object.f);
+  assertSerialize('null', object.f);
   assertSerialize('{"s":"string value","b":true,"i":100}', object);
 }