Implement the possibility to override the value of any properties tha…

[Aymenworks]

Since I need this feature, I decided to continue this idea : https://github.com/JohnSundell/Wrap/issues/23.

So this PR Implement the possibility to override the value of any properties that contains nil values

Here is an example of use and output :

struct SimpleTrip {
    let name: String
    let numberOfDays: Int?
    let numberOfSpots: Int?
    let updatedDate: Date?
    let endDate: Date?
}

extension SimpleTrip: WrapCustomizable {
    func keyForWrapping(propertyName: String) -> String? {
        if propertyName == "updatedDate" {
            return "newUpdatedDate"
        } else if propertyName == "endDate" {
            return "newEndDate"
        }
        
        return propertyName
    }

    func fallbackValueForProperty(named propertyName: String) -> Any? {
        if propertyName == "updatedDate" {
            return Optional<Any?>(nil)
        } else if propertyName == "numberOfSpots" {
            return 0
        } else if propertyName == "endDate" {
            return Date()
        }
        
        return nil
    }
}

let value = SimpleTrip(name: "Name1", numberOfDays: nil, numberOfSpots: nil, updatedDate: nil, endDate: nil)

let dictionary: WrappedDictionary = try Wrap(value)

Output of dictionary:

["name": "Name1", "newUpdatedDate": nil, "numberOfSpots": 0, "newEndDate": 2017-09-08 14:53:50 +0000]

[Aymenworks]

Ooops !

It seems it's a duplicate of #36.

But it also seems that there is no news from this PR.

What do we do @JohnSundell ?

[Aymenworks]

It seems that my changes worked on a playground but not in the real project, strange..

I'll use the #36 for now.