Added refresh method

[Pimm]

API

Objects returned by the API now have a refresh method, which returns a fresh copy of said object.

let payment = await mollieClient.payments.get('tr_44aKxzEbr8');
// Later…
payment = await payment.refresh();

This is a more JavaScript-like interface to the "self" link included in objects returned by the Mollie server. It obsoletes that "self" link, allowing us to hide it from users in the next major release.

Internal

Previously, every binder encapsulated the logic for transforming the objects it received. To facilitate the refresh method ‒ but especially other future links, such as a getOrder method ‒ all transformation logic now lives inside the TransformingNetworkClient and is shared across binders.