Issue Description:

Decoding JSON with JSONDecoder into a Codable conforming class with properties with default values doesn't work as expected. If the JSON has those properties set with a value, this value gets ignored and the default value of the property gets set.

For example, lets say I have this class:

public class Example: Codable {

{{ public let name: String = ""}}

{{}}}

And this JSON inside a Data property called data:

{name: "Some Name"}

When doing the following, the 'name' property has a value of "" and not "Some Name".

let example = try? JSONDecoder().decode(Example.self, from: data)

If I remove the default value and move it into an initializer, decoding works as expected and the 'name' property ends up with a value of "Some Name":

public class Example: Codable {
public let name: String
init() { name = "" }
}