Move open method in downloader out

Sources/ImageDownloader.swift

@@ -247,33 +247,33 @@ open class ImageDownloader: NSObject {
         barrierQueue.sync { fetchLoad = fetchLoads[url] }
         return fetchLoad
     }
-}
-
-// MARK: - Download method
-extension ImageDownloader {
-    /**
-    Download an image with a URL and option.
     
-    - parameter url:               Target URL.
-    - parameter options:           The options could control download behavior. See `KingfisherOptionsInfo`.
-    - parameter progressBlock:     Called when the download progress updated.
-    - parameter completionHandler: Called when the download progress finishes.
-
-    - returns: A downloading task. You could call `cancel` on it to stop the downloading process.
-    */
+    /**
+     Download an image with a URL and option.
+     
+     - parameter url:               Target URL.
+     - parameter options:           The options could control download behavior. See `KingfisherOptionsInfo`.
+     - parameter progressBlock:     Called when the download progress updated.
+     - parameter completionHandler: Called when the download progress finishes.
+     
+     - returns: A downloading task. You could call `cancel` on it to stop the downloading process.
+     */
     @discardableResult
     open func downloadImage(with url: URL,
-                             options: KingfisherOptionsInfo? = nil,
-                       progressBlock: ImageDownloaderProgressBlock? = nil,
-                   completionHandler: ImageDownloaderCompletionHandler? = nil) -> RetrieveImageDownloadTask?
+                            options: KingfisherOptionsInfo? = nil,
+                            progressBlock: ImageDownloaderProgressBlock? = nil,
+                            completionHandler: ImageDownloaderCompletionHandler? = nil) -> RetrieveImageDownloadTask?
     {
         return downloadImage(with: url,
-                retrieveImageTask: nil,
-                          options: options,
-                    progressBlock: progressBlock,
-                completionHandler: completionHandler)
+                             retrieveImageTask: nil,
+                             options: options,
+                             progressBlock: progressBlock,
+                             completionHandler: completionHandler)
     }
-    
+}
+
+// MARK: - Download method
+extension ImageDownloader {
     func downloadImage(with url: URL,
               retrieveImageTask: RetrieveImageTask?,
                         options: KingfisherOptionsInfo?,