[IndexedDB] IDBObjectStore.index() is missing step to throw if transaction is finished (#37)

[IDBTransaction.objectStore()](https://w3c.github.io/IndexedDB/#dom-idbtransaction-objectstore) has:

>If transaction has finished, throw an InvalidStateError exception.

But [IDBObjectStore.index()](https://w3c.github.io/IndexedDB/#dom-idbobjectstore-index) does not have a corresponding case.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/37