Cannot validate form when keys of collection in data are non consecutive

As the `InputFilter` tests the keys of the `validationGroup` to the keys of the `data`, the `prepareValidationGroup` should in case of a `Collection` use the keys of the corresponding `data` when multiplying that validationGroup to match the amount of Collection items in `data`.
Most of the time the keys of a collection inside `data` are consecutive, but not per se. The test attached to this PR shows this more clearly.