Fix: allow data size = exact multiple of threshold

Hello @evansolomon, 

Please review the following commits I made in branch 'tyler-exact-threshold'.

4fd826773b2ee69a9de8cb34ab2816a91cc30513 (2013-09-28 00:02:31 +0000)
Fix: allow data size = exact multiple of threshold

Check list:


R=@evansolomon