adding IsInitialized property to nullable types

[honibis]

When setting database from json web services, we only set value fields that should be updated and ignore other fields by simply not setting in the json. When deserilazed into nullable objects, currently we can not distinguish if the field was set to null or was not defined at all, since they both deserialize to null value. If there was an IsInitialized property at nullable types like HasValue, we could determine if we should set the value to null in db or just ignore.