Issue 463 - fix to show dates in collection tile

In #463 I reported and fixed the dates which are shown in the results of the collectio n tile.
I think is easier to use the built-in function ``callable``, so you don't have to import anything else:

```python
return brain.Date() if callable(brain.Date) else return brain.Date
```

also, remember to add a note on the code on why you're doing this so we do not forget it later.