change willChangeValueForKey to didChangeValueForKey on UIScrollView + MJRefresh.m on line 100

MJRefresh/UIScrollView+MJRefresh.m

@@ -97,7 +97,7 @@ - (void)setReloadDataBlock:(void (^)(NSInteger))reloadDataBlock
 {
     [self willChangeValueForKey:@"reloadDataBlock"]; // KVO
     objc_setAssociatedObject(self, &MJRefreshReloadDataBlockKey, reloadDataBlock, OBJC_ASSOCIATION_COPY_NONATOMIC);
-    [self willChangeValueForKey:@"reloadDataBlock"]; // KVO
+    [self didChangeValueForKey:@"reloadDataBlock"]; // KVO
 }
 
 - (void (^)(NSInteger))reloadDataBlock