fix(splito-api-binding): ensure latest splits are saved and add tests

[celiawaggoner]

modify behavior in getSplits so that we don't save splits that have been archived after the initial request.

For Splits you'll receive again the full split object for each split that received changes since the last fetch. It's safe to assume that you can just overwrite the representation you had for that specific split, being the name of the split a unique id.

In case of the edge case, you'll hit it with -1 and again with a different value. The second request can come either empty (because you had up to date information) or with the updates that were not propagated yet.