This needs to be copied by value, not by reference, as the `localStorage` keys added below pollute the `data.keys` array, which are then persisted when `save` is called on any entry. This causes `has` to return true, because it had a `localStorage` key at one point, but then `get` fails because the value doesn't exist.

This method should only reflect the state of the `data.keys` list, and the **current** state of the `localStorage` keys, it shouldn't save the state of the latter, as they can chance outside `fifo`'s control.