JSON MarshallableDocument Raw function

If an object is created programmatically (e.g. with a call to `NewPolicyCertificate()` to create a Policy), the JSONField is not set accordingly. Then if a call to `Raw()` is done, nothing is returned.
Same applies when a field of the object is changed.
Make the object read-only and cache the JSON in a variable, or make `Raw()` to always derive the JSON at call time.
