Requests with null value fields

[XtraSonic]

JSONObject, which is used to create the payload of the request, does not put fields in the payload if they have the value (java) null. If we want to add a field with the value null, it has to have the value "JSONObject.NULL" (see the json object documentation)

There are two ways we can handle this:

1. Create a NullFieldModel class which gets a name and always returns the value JSONObject.NULL. When the user wants to set a field with a null value, it will create a new instance of this class and set it in the entity model.

2. Treat all null values as JSONObject.NULL. This might have a backwards compatibility impact since now some fields which were previously ignored, will be added to the request and will have the value null.

I personally think that the first option is better.