Present null for property with nil value

[levanlongktmt]

Currently, all property with nil value will be ignore when serialized. But in some case, it need present a null text (for example: send request update data of record to server).
So I add a little bit codes to help user can add property which should be present null when value of it is nil.

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[emrcftci]

Maybe we will use like this ->
useNullValueIfNeeded = mapper.propertyPresentNullValue(key: Int(bitPattern: info.address))

just a proposal