Respect user passing nil for downloader

Sources/ImageDownloader.swift

@@ -140,8 +140,10 @@ public protocol ImageDownloaderDelegate: class {
      - parameter data:       Downloaded data.
      - parameter url:        URL of the original request URL.
      
+     - returns: The data from which Kingfisher should use to create an image.
+     
      - Note: This callback can be used to preprocess raw image data
-             before creation of UIImage instance (i.e. decrypting).
+             before creation of UIImage instance (i.e. decrypting or verification).
      */
     func imageDownloader(_ downloader: ImageDownloader, didDownload data: Data, for url: URL) -> Data?
 }
@@ -551,11 +553,13 @@ class ImageDownloaderSessionHandler: NSObject, URLSessionDataDelegate, Authentic
             
             self.cleanFetchLoad(for: url)
             
-            let data: Data
-            if let modifiedData = downloader.delegate?.imageDownloader(downloader, didDownload: fetchLoad.responseData as Data, for: url) {
-                data = modifiedData
+            let data: Data?
+            let fetchedData = fetchLoad.responseData as Data
+            
+            if let delegate = downloader.delegate {
+                data = delegate.imageDownloader(downloader, didDownload: fetchedData, for: url)
             } else {
-                data = fetchLoad.responseData as Data
+                data = fetchedData
             }
             
             // Cache the processed images. So we do not need to re-process the image if using the same processor.
@@ -570,7 +574,7 @@ class ImageDownloaderSessionHandler: NSObject, URLSessionDataDelegate, Authentic
                 let processor = options.processor
                 
                 var image = imageCache[processor.identifier]
-                if image == nil {
+                if let data = data, image == nil {
                     image = processor.process(item: .data(data), options: options)
                     // Add the processed image to cache. 
                     // If `image` is nil, nothing will happen (since the key is not existing before).

Tests/KingfisherTests/ImageDownloaderTests.swift

@@ -381,6 +381,31 @@ class ImageDownloaderTests: XCTestCase {
 
         waitForExpectations(timeout: 5, handler: nil)
     }
+    
+    func testDownloadedDataCouldBeModified() {
+        let expectation = self.expectation(description: "wait for downloading image")
+        
+        let URLString = testKeys[0]
+        _ = stubRequest("GET", URLString).andReturn(200)?.withBody(testImageData)
+        
+        let url = URL(string: URLString)!
+        
+        downloader.delegate = self
+        downloader.downloadImage(with: url) { image, error, imageURL, data in
+            XCTAssertNil(image)
+            XCTAssertNotNil(error)
+            XCTAssertEqual(error?.code, KingfisherError.badData.rawValue)
+            self.downloader.delegate = nil
+            expectation.fulfill()
+        }
+        waitForExpectations(timeout: 5, handler: nil)
+    }
+}
+
+extension ImageDownloaderTests: ImageDownloaderDelegate {
+    func imageDownloader(_ downloader: ImageDownloader, didDownload data: Data, for url: URL) -> Data? {
+        return nil
+    }
 }
 
 class URLModifier: ImageDownloadRequestModifier {