Update existing object with json data

Currently there is no way to insert json data to an object that already exists.

It would be great to be able to do as below (use case for Realm):

``` java
@JsonObject
public class MyObject extends RealmObject {
@JsonField
@JsonObjectId
String id;

@JsonField
int someField;

@JsonField
String anotherField;

@OnJsonIdRead
MyObject onJsonIdRead(String id){
return MyRealmManager.getRealm().where(MyObject.class).equalTo("id", id).findFirst();
}
}
```

It would solve a problem with server returning different versions of json i.e. we already have `id` and `anotherField`, but in some call server returns only `id` and `someField`. Realm can't help with it's `createOrUpdateAllFromJson()`.

Start by reviewing the Realm createOrUpdateAllFromJson() use case and the @OnJsonIdRead example in the issue. Define how an existing object is found by its JSON id and how partial JSON updates preserve fields omitted from the response; done means this behavior is supported without losing existing values.

Written by the indexing model from the issue text.