Fix typo: retrieveImagetask -> retrieveImageTask

Signed-off-by: mrahmiao <mrahmiao@gmail.com>

Kingfisher/ImageDownloader.swift

@@ -74,14 +74,14 @@ public extension ImageDownloader {
         completionHandler: ImageDownloaderCompletionHandler?)
     {
         downloadImageWithURL(URL,
-            retrieveImagetask: nil,
+            retrieveImageTask: nil,
                       options: options,
                 progressBlock: progressBlock,
             completionHandler: completionHandler)
     }
     
     internal func downloadImageWithURL(URL: NSURL,
-                       retrieveImagetask: RetrieveImageTask?,
+                       retrieveImageTask: RetrieveImageTask?,
                                  options: KingfisherManager.Options,
                            progressBlock: ImageDownloaderProgressBlock?,
                        completionHandler: ImageDownloaderCompletionHandler?)
@@ -97,7 +97,7 @@ public extension ImageDownloader {
             
             fetchLoad.shouldDecode = options.shouldDecode
             
-            retrieveImagetask?.downloadTask = task
+            retrieveImageTask?.downloadTask = task
         }
     }
     

Kingfisher/KingfisherManager.swift

@@ -153,7 +153,7 @@ public class KingfisherManager {
                         completionHandler: CompletionHandler?,
                                   options: Options)
     {
-        downloader.downloadImageWithURL(URL, retrieveImagetask: retrieveImageTask, options: options, progressBlock: { (receivedSize, totalSize) -> () in
+        downloader.downloadImageWithURL(URL, retrieveImageTask: retrieveImageTask, options: options, progressBlock: { (receivedSize, totalSize) -> () in
             progressBlock?(receivedSize: receivedSize, totalSize: totalSize)
             return
         }) { (image, error, imageURL) -> () in