Update default key formatter

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

The JSON:API specification now recommends "Member names SHOULD be camel-cased." As such, we should update the default key formatter.

• Update code
• Update docs

Individua il formatter delle chiavi predefinito attuale e verifica le indicazioni sul formato delle chiavi di JSON:API collegate nell’issue. Conferma che il formatter produca nomi dei membri camel-cased, quindi aggiorna l’implementazione e la documentazione referenziata se entrambe rientrano nell’ambito.