IDL: allow optional values to be undefined in overload resolution

LayoutTests/storage/indexeddb/index-basics-workers-expected.txt

 [Worker] Test the basics of IndexedDB's webkitIDBIndex.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 Starting worker: resources/index-basics.js
 [Worker] indexedDB = self.indexedDB || self.webkitIndexedDB || self.mozIndexedDB || self.msIndexedDB || self.OIndexedDB;
 [Worker] 
 [Worker] dbname = "index-basics.js"
 [Worker] indexedDB.deleteDatabase(dbname)
 [Worker] indexedDB.open(dbname)
 [Worker] 
 [Worker] prepareDatabase():
 [Worker] db.createObjectStore('storeName', null)
 [Worker] store.createIndex('indexName', 'x')
 [Worker] store.createIndex('indexName2', 'y', {unique: false})
 [Worker] store.createIndex('zIndex', 'z', {unique: true})
+PASS [Worker] store.createIndex('index4', 'path', undefined) did not throw exception.
 PASS [Worker] indexObject2.unique is false
 PASS [Worker] indexObject3.unique is true
 [Worker] Expecting TypeError exception from store.createIndex('failureIndex', 'zzz', true)
 PASS [Worker] Exception was thrown.
 PASS [Worker] store.createIndex('failureIndex', 'zzz', true) threw TypeError: Failed to execute 'createIndex' on 'IDBObjectStore': No function was found that matched the signature provided.
 [Worker] Expecting TypeError exception from store.createIndex('failureIndex', 'zzz', 'string')
 PASS [Worker] Exception was thrown.
 PASS [Worker] store.createIndex('failureIndex', 'zzz', 'string') threw TypeError: Failed to execute 'createIndex' on 'IDBObjectStore': No function was found that matched the signature provided.
 PASS [Worker] 'name' in indexObject is true
 PASS [Worker] indexObject.name is "indexName"
@@ skipping 107 matching lines @@
 [Worker] Passing an invalid key into indexObject.getKey({}).
 [Worker] Expecting exception from indexObject.getKey({})
 PASS [Worker] Exception was thrown.
 PASS [Worker] code is 0
 PASS [Worker] ename is 'DataError'
 [Worker] Exception message: Failed to execute 'getKey' on 'IDBIndex': The parameter is not a valid key.
 PASS successfullyParsed is true
 
 TEST COMPLETE