     /// The duration before the download is timeout. Default is 15 seconds.
     public var downloadTimeout: NSTimeInterval = 15.0
     
-    /// A set of trusted hosts when receiving server trust challenges. A challenge with host name contained in this set will be ignored. You can use this set to specify the self-signed site.
+    /// A set of trusted hosts when receiving server trust challenges. A challenge with host name contained in this set will be ignored. You can use this set to specify the self-signed site. It only will be used if you don't specify the `authenticationChallengeResponder`. If `authenticationChallengeResponder` is set, this property will be ignored and the implemention of `authenticationChallengeResponder` will be used instead.
     public var trustedHosts: Set<String>?
     
     /// Use this to set supply a configuration for the downloader. By default, NSURLSessionConfiguration.ephemeralSessionConfiguration() will be used. You could change the configuration before a downloaing task starts. A configuration without persistent storage for caches is requsted for downloader working correctly.

                dataTask.resume()
                
                // Hold self while the task is executing.
                self.sessionHandler?.downloadHolder = self
            }
            
            fetchLoad.downloadTaskCount += 1

            return
        }
        
        downloader.authenticationChallengeResponder?.downloder(downloader, didReceiveChallenge: challenge, completionHandler: completionHandler)
    }
    
    private func callbackWithImage(image: Image?, error: NSError?, imageURL: NSURL, originalData: NSData?) {
