fix for last being broken with reorder descending

This is very speculative so I'd appreciate if @bigxiang could take a look.  You can see my failing test.  I don't really understand the changes (apparently related to reorder(nil)). that may have introduced this problem to begin with.  It would probably make sense to run the entire activerecord test suite against squeel just to make sure this fix is safe.  Don't know if that's easy to do.