Added missing labels to `Validation` typealias in `DownloadRequest` extension.

Source/Validation.swift

@@ -222,7 +222,7 @@ extension DataRequest {
 extension DownloadRequest {
     /// A closure used to validate a request that takes a URL request, a URL response, a temporary URL and a 
     /// destination URL, and returns whether the request was valid.
-    public typealias Validation = (URLRequest?, HTTPURLResponse, _ temporary: URL?, _ destination: URL?) -> ValidationResult
+    public typealias Validation = (_ request: URLRequest?, _ response: HTTPURLResponse, _ temporary: URL?, _ destination: URL?) -> ValidationResult
 
     /// Validates the request, using the specified closure.
     ///