     @discardableResult
     public func responseJSONDecodable<T: Decodable>(
         queue: DispatchQueue? = nil,
-        decoder: JSONDecoder? = JSONDecoder(),
+        decoder: JSONDecoder = JSONDecoder(),
         completionHandler: @escaping (DataResponse<T>) -> Void)
         -> Self
     {

    ///
    /// - returns: A JSON Decodable object response serializer.
    public static func JSONDecodableResponseSerializer<T: Decodable>(
        decoder: JSONDecoder? = JSONDecoder())
        -> DownloadResponseSerializer<T>
    {
        return DownloadResponseSerializer { _, response, fileURL, error in

    @discardableResult
    public func responseJSONDecodable<T: Decodable>(
        queue: DispatchQueue? = nil,
        decoder: JSONDecoder = JSONDecoder(),
        completionHandler: @escaping (DownloadResponse<T>) -> Void)
        -> Self
    {
