Fix ExclusiveStartKey on query FakeDynamo

Hello @x-ma, 

Please review the following commits I made in branch 'nick-query2'.

3a55f7a55a59fdf44f18611cd79479f068f52ee5 (2013-11-19 13:07:53 -0500)
Fix ExclusiveStartKey on query FakeDynamo

R=@x-ma