SDWebImageDownloaderOperation: call endBackgroundTask after operation is finishe…

…d so that beginBackgroundTaskWithExpirationHandler and endBackgroundTask are balanced

@@ -108,6 +108,13 @@
             self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}], YES);
         }
     }
+
+#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
+    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
+        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskId];
+        self.backgroundTaskId = UIBackgroundTaskInvalid;
+    }
+#endif
 }
 
 - (void)cancel {