Fixed issue #2573

Hi,

I was able to reproduce / confirm the issue described in #2573 by generating 30k+ entries with the same HashKey and seeing discrepancies in the count results between Table.count() and query_count.
The fix was tested on a local DynamoDB.


Note: This is my first contribution for boto, let me know if there's anything I should improve. Thanks!
This line and the one above are a bit long - can you stick with PEP8 (80 character line widths)?