修复切换tab栏时，header没有重置位置的bug (#1271)

- recovery for changing view state

MJRefresh/Base/MJRefreshHeader.m

@@ -48,6 +48,13 @@ - (void)placeSubviews
     self.mj_y = - self.mj_h - self.ignoredScrollViewContentInsetTop;
 }
 
+- (void)willMoveToWindow:(UIWindow *)newWindow {
+    
+    if (!newWindow && self.isRefreshing) {
+        [self endRefreshing];
+    }
+}
+
 - (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
 {
     [super scrollViewContentOffsetDidChange:change];