Fixed a bug with CassandraSlicePredicateQueryable (Count operations)

Fixed a null reference bug with CassandraSlicePredicateQueryable queries.

If you run a query like the one below using FluentCassandra 1.2.1

````
var userSessions =
userSessionsCf.Get(rowKey)
.StartWithColumn(start.Ticks)
.TakeUntilColumn(start.AddDays(1).Ticks - 1)
 CountColumns();
````

You will receive an error thrown upon execution of the request due to the CountColumn operation's ColumnFamily field being null. 

This patch addresses that and causes this query to work as expected.