Fixed issue where error occurs when deleting a session that does not exist.

Author: David Massad

lib/aws/session_store/dynamo_db/locking/base.rb

@@ -143,7 +143,7 @@ def data_unchanged?(env, session)
 
     # Expected attributes
     def expected_attributes(sid)
-      { :expected => {@config.table_key => {:value => {:s => sid}, :exists => true}} }
+      { :expected => {@config.table_key => {:value => {:s => sid}, :exists => false}} }
     end
 
     # Attributes to be retrieved via client