Fix viewWillAppear Not Refresh

MJRefreshExample/MJRefreshExample/MJRefresh/MJRefreshBaseView.m

@@ -147,6 +147,9 @@ - (void)beginRefreshing
         } else {
     #warning 不能调用set方法
             _state = MJRefreshStateWillRefreshing;
+            
+#warning 为了保证在viewWillAppear等方法中也能刷新
+            [self setNeedsDisplay];
         }
     }
 }