[Collection] Use PropertyPath

[Taluu]

As Symfony's ProperyAccess was integrated with the CollectionSnapshot (cf #22), and as in a collection the whole objets should look alike (collection of objects, of arrays, ...), we should not try to modify the primary parameter.

If the snapshot is used with a collection of arrays, then we should either have a PropertyPath directly fed to the pkey argument, or a valid property path either way [key]. If it is a collection of objects, then either a PropertyPath object, either a key path.

This should be way more flexible than it actually is right now, instead of doing a wild guess depending on the types of each elements fed into the collection, which could be wrong.