for iOS7 call downloadProgress block

AFNetworking/AFURLSessionManager.m

@@ -222,7 +222,7 @@ typedef void (^AFURLSessionTaskCompletionHandler)(NSURLResponse *response, id re
 }
 
 - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
-    if ([object isKindOfClass:[NSURLSessionTask class]]) {
+    if ([object isKindOfClass:[NSURLSessionTask class]] || [object isKindOfClass:[NSURLSessionDownloadTask class]]) {
         if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesReceived))]) {
             self.downloadProgress.completedUnitCount = [change[@"new"] longLongValue];
         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))]) {