Change `isRunning` on manager from method to property

SDWebImage/SDWebImageManager.h

@@ -134,6 +134,11 @@ SDWebImageManager.sharedManager.cacheKeyFilter = ^(NSURL * _Nullable url) {
  */
 @property (nonatomic, copy, nullable) SDWebImageCacheSerializerBlock cacheSerializer;
 
+/**
+ * Check one or more operations running
+ */
+@property (nonatomic, assign, readonly, getter=isRunning) BOOL running;
+
 /**
  * Returns global shared manager instance.
  */
@@ -208,11 +213,6 @@ SDWebImageManager.sharedManager.cacheKeyFilter = ^(NSURL * _Nullable url) {
  */
 - (void)cancelAll;
 
-/**
- * Check one or more operations running
- */
-- (BOOL)isRunning;
-
 /**
  *  Async check if image has already been cached
  *