Fix the getCollectionDate... functions

- `getCollectionDateLastModified()` and `getCollectionDateAdded()` returned not localized dates when using a mask: let's localize them

- `getCollectionDatePublic()` has the same function as `getCollectionDateLastModified()` and `getCollectionDateAdded()`, but it has a worst implementation (a useless call to the localization when we don't use a mask, and uses a hard coded string to represent the *"don't use a mask"* case). So, let's use for `getCollectionDatePublic()` the same approach used by the other two functions