Fix date serializer

[parayab]

This PR changes the library behaviour, so it might break apps using it.

Before a Movement serialized like this:

amount: 7844717
currency: "CLP"
description: "Nihil aspernatur dignissimos est."
id: "mov_ZxENnDHvJAxreJkK"
postDate: {}
transactionDate: null
}

postDate and transactionDate, if not null, are serialized as empty objects.

Now a Movement is serialized like this:

amount: 7844717
currency: "CLP"
description: "Nihil aspernatur dignissimos est."
id: "mov_ZxENnDHvJAxreJkK"
postDate: "2021-05-11T00:00:00.000Z"
transactionDate: null
}

postDate and transactionDate, if not null, are serialized as strings.

[nateare]

LGTM

Before merging we should change the commit message to follow the same standard the public repos follow

[lezorich]

LGTM