Fix BatchGetItem paginated request data format.

[anthony-ffs]

This fixes the format of paginated BatchGetItem request using Table.getItems.

Without this change, the first request would succeed, but subsequent paginated requests would fail with MissingRequiredParameter: Missing required key 'RequestItems' in params. This is because the requested keys were sent as the request, rather than being placed on request.RequestItems.

Note that a large number of items must be requested in order for BatchGetItem requests to be paginated. Even with 5000 items requested DynamoDB would sometimes return them all in one response.