Fix nil @_start_transaction_state variable.

Which was throwing an `undefined method '[]' for nil:NilClass` error in Rails 4.0.2.

I noticed that these instance variables from `ActiveRecord::Core#init_internals` aren't being set either. But they haven't given me trouble so I didn't include them.

```
@txn = nil
@_start_transaction_state = {}
@reflects_state = [false]
@destroyed_by_association = nil
```