Made initializers for both default responses public and added metrics parameter.

This change allows default responses to be rebuilt in client APIs as necessary. Certain use cases would be wrapping up errors in an external error type as well as modifying the data payload for a downstream response handler.

Source/Response.swift

@@ -43,7 +43,23 @@ public struct DefaultDataResponse {
 
     var _metrics: AnyObject?
 
-    init(request: URLRequest?, response: HTTPURLResponse?, data: Data?, error: Error?, timeline: Timeline = Timeline()) {
+    /// Creates a `DefaultDataResponse` instance from the specified parameters.
+    ///
+    /// - Parameters:
+    ///   - request:  The URL request sent to the server.
+    ///   - response: The server's response to the URL request.
+    ///   - data:     The data returned by the server.
+    ///   - error:    The error encountered while executing or validating the request.
+    ///   - timeline: The timeline of the complete lifecycle of the request. `Timeline()` by default.
+    ///   - metrics:  The task metrics containing the request / response statistics. `nil` by default.
+    public init(
+        request: URLRequest?,
+        response: HTTPURLResponse?,
+        data: Data?,
+        error: Error?,
+        timeline: Timeline = Timeline(),
+        metrics: AnyObject? = nil)
+    {
         self.request = request
         self.response = response
         self.data = data
@@ -148,14 +164,26 @@ public struct DefaultDownloadResponse {
 
     var _metrics: AnyObject?
 
-    init(
+    /// Creates a `DefaultDownloadResponse` instance from the specified parameters.
+    ///
+    /// - Parameters:
+    ///   - request:        The URL request sent to the server.
+    ///   - response:       The server's response to the URL request.
+    ///   - temporaryURL:   The temporary destination URL of the data returned from the server.
+    ///   - destinationURL: The final destination URL of the data returned from the server if it was moved.
+    ///   - resumeData:     The resume data generated if the request was cancelled.
+    ///   - error:          The error encountered while executing or validating the request.
+    ///   - timeline:       The timeline of the complete lifecycle of the request. `Timeline()` by default.
+    ///   - metrics:        The task metrics containing the request / response statistics. `nil` by default.
+    public init(
         request: URLRequest?,
         response: HTTPURLResponse?,
         temporaryURL: URL?,
         destinationURL: URL?,
         resumeData: Data?,
         error: Error?,
-        timeline: Timeline = Timeline())
+        timeline: Timeline = Timeline(),
+        metrics: AnyObject? = nil)
     {
         self.request = request
         self.response = response