IDL: allow optional values to be undefined in overload resolution

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

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