Remove FindOne from Update to allow atomic implementations.

[sharpner]

Since jsonapi 1.0 we use PATCH to update entities.

We need to refactor the handleUpdate accordingly and keep links and relations in mind, since they can also get updated.

[Linuturk]

I think I might be impacted by this right now. I have a struct that contains a map[string]string, and the updated map from the client request is being merged with any existing map in the database. This means I can't ever remove items from the map because api2go is merging them before calling my Update() method. Is this behavior consistent with the problem you are reporting? Is there a work around for this?