Add default property into response model

[garrettrathbone]

Currently, if a property that is in the model doesn't exist in the returned results set, it returns nothing, regardless of if a default is provided or not. It seems rational to not return a value in the model that doesn't exist, which allows someone to understand what is coming from the API or not. But I think there are benefits to allowing someone to set default on a property and allow the model to return that in the situation that the value wasn't set in the original response. By checking whether a default is set in general, regardless of if it is truthy, falsey, or whatever else, this supports backwards compatibility with the option to 'opt in' to setting a default for a property.

[garrettrathbone]

Pull Request made here. #187