Fix bug when setting both sortProperties and sortAscending in SortableMixin

I'm setting both sortProperties and sortAscending using one call

```js
sortedArrayController.setProperties({ sortProperties: ['id'], sortAscending: true});
```

The arrangedContent gets sorted with new value for sortAscending, and then gets reversed, when the sortAscendingDidChange observer kicks in. To prevent this, _lastSortAscending needs to be set when the array is sorted


<!---
@huboard:{"order":4708.0,"custom_state":"blocked"}
-->