CBQE-0: BatchedLoadDocumentsTask._create_batch hangs on AttributeError

Change-Id: I31e35e7c1942db719c1aa010e8be3b1179074c23
Reviewed-on: http://review.couchbase.org/26374
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>