batch_size 1 prevents iteration over query results

If you set batch_size to 1, you cannot use ```each ``` to iterate over a collection. It looks like a cursor_id is never assigned, so ```while more?``` is false in ```Cursor#each```. Is this a moped or a mongodb issue? I realise it's a bit of an odd thing to do, but I'm adding Mongoid support to a gem whose test suite is setting batch_size to 1 for a particular test case.