Issue Description:

The following code successfully creates an HTTPCookie on Darwin, whereas on Linux the initializer returns nil:

let cookie = HTTPCookie(properties: [.domain: ".", .path: "/", .name: "Davey".dropLast(), .value: "Jonesy".dropLast()])
if let cookie = cookie {
    print("\(cookie)")
} else {
    print("No cookie :(")
}

The initializer takes values as Any, so this code compiles, but fails silently. The initializer in SCLF is trying to cast the values to String:

Is it expected that you should be able to pass a Substring here, and should the cast be to something like StringProtocol instead?