Include nulls with JSONEncoder

How can I tell JSONEncoder that it SHOULD include they property key even when the optional value is nil? In the generated JSON I want a null. For example:

struct Foo : Codable {
    var id = 7
    var foo: String?
}

let data = try! JSONEncoder().encode(Foo())
String(data: data, encoding: .utf8)

That just prints out

{"id":7}

when what I really want is:

{"id":7,"foo":null}