Add header as subview when set new.

Signed-off-by: 庞博 <bopang@sohu-inc.com>

MJRefreshExample/MJRefreshExample/MJRefresh/UIScrollView+MJRefresh.m

@@ -46,7 +46,6 @@ - (MJRefreshLegendHeader *)addLegendHeaderWithRefreshingBlock:(void (^)())block
 - (MJRefreshLegendHeader *)addLegendHeader
 {
     MJRefreshLegendHeader *header = [[MJRefreshLegendHeader alloc] init];
-    [self addSubview:header];
     self.header = header;
     
     return header;
@@ -82,7 +81,6 @@ - (MJRefreshGifHeader *)addGifHeaderWithRefreshingTarget:(id)target refreshingAc
 - (MJRefreshGifHeader *)addGifHeader
 {
     MJRefreshGifHeader *header = [[MJRefreshGifHeader alloc] init];
-    [self addSubview:header];
     self.header = header;
     
     return header;
@@ -126,6 +124,8 @@ - (void)setHeader:(MJRefreshHeader *)header
                                  header,
                                  OBJC_ASSOCIATION_ASSIGN);
         [self didChangeValueForKey:@"header"];
+        
+        [self addSubview:header];
     }
 }