Adds setter to override default SDWebImageDownloaderOperation.

SDWebImage/SDWebImageDownloader.h

@@ -139,6 +139,16 @@ typedef NSDictionary *(^SDWebImageDownloaderHeadersFilterBlock)(NSURL *url, NSDi
  */
 - (NSString *)valueForHTTPHeaderField:(NSString *)field;
 
+/**
+ * Sets a subclass of `SDWebImageDownloaderOperation` as the default
+ * `NSOperation` to be used each time SDWebImage constructs a request
+ * operation to download an image.
+ *
+ * @param operationClass The subclass of `SDWebImageDownloaderOperation` to set 
+ *        as default. Passing `nil` will revert to `SDWebImageDownloaderOperation`.
+ */
+- (void)setOperationClass:(Class)operationClass;
+
 /**
  * Creates a SDWebImageDownloader async downloader instance with a given URL
  *

SDWebImage/SDWebImageDownloader.m

@@ -20,6 +20,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
 
 @property (strong, nonatomic) NSOperationQueue *downloadQueue;
 @property (weak, nonatomic) NSOperation *lastAddedOperation;
+@property (assign, nonatomic) Class operationClass;
 @property (strong, nonatomic) NSMutableDictionary *URLCallbacks;
 @property (strong, nonatomic) NSMutableDictionary *HTTPHeaders;
 // This queue is used to serialize the handling of the network responses of all the download operation in a single queue
@@ -63,6 +64,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
 
 - (id)init {
     if ((self = [super init])) {
+        _operationClass = [SDWebImageDownloaderOperation class];
         _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
         _downloadQueue = [NSOperationQueue new];
         _downloadQueue.maxConcurrentOperationCount = 2;
@@ -104,6 +106,10 @@ static NSString *const kCompletedCallbackKey = @"completed";
     return _downloadQueue.maxConcurrentOperationCount;
 }
 
+- (void)setOperationClass:(Class)operationClass {
+    _operationClass = operationClass ?: [SDWebImageDownloaderOperation class];
+}
+
 - (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
     __block SDWebImageDownloaderOperation *operation;
     __weak SDWebImageDownloader *wself = self;
@@ -124,7 +130,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
         else {
             request.allHTTPHeaderFields = wself.HTTPHeaders;
         }
-        operation = [[SDWebImageDownloaderOperation alloc] initWithRequest:request
+        operation = [[wself.operationClass alloc] initWithRequest:request
                                                           options:options
                                                          progress:^(NSInteger receivedSize, NSInteger expectedSize) {
                                                              SDWebImageDownloader *sself = wself;