IDL: allow optional values to be undefined in overload resolution

LayoutTests/storage/indexeddb/resources/index-basics.js

 if (this.importScripts) {
     importScripts('../../../resources/js-test.js');
     importScripts('shared.js');
 }
 
 description("Test the basics of IndexedDB's webkitIDBIndex.");
 
 indexedDBTest(prepareDatabase);
 function prepareDatabase(evt)
 {
     preamble(evt);
     db = event.target.result;
     event.target.transaction.onabort = unexpectedAbortCallback;
     self.store = evalAndLog("db.createObjectStore('storeName', null)");
     self.indexObject = evalAndLog("store.createIndex('indexName', 'x')");
     self.indexObject2 = evalAndLog("store.createIndex('indexName2', 'y', {unique: false})");
     self.indexObject3 = evalAndLog("store.createIndex('zIndex', 'z', {unique: true})");
+    shouldNotThrow("store.createIndex('index4', 'path', undefined)");
     shouldBeFalse("indexObject2.unique");
     shouldBeTrue("indexObject3.unique");
     evalAndExpectExceptionClass("store.createIndex('failureIndex', 'zzz', true)", "TypeError");
     evalAndExpectExceptionClass("store.createIndex('failureIndex', 'zzz', 'string')", "TypeError");
     addData();
 }
 
 function addData()
 {
     shouldBeTrue("'name' in indexObject");
@@ skipping 287 matching lines @@
     shouldBe("event.target.result", "2");
 
     debug("Passing an invalid key into indexObject.get({}).");
     evalAndExpectException("indexObject.get({})", "0", "'DataError'");
 
     debug("Passing an invalid key into indexObject.getKey({}).");
     evalAndExpectException("indexObject.getKey({})", "0", "'DataError'");
 
     finishJSTest();
 }