MJRefresh的triggerAutomaticallyRe

集成MJRefresh出现的问题：

let header = MJRefreshNormalHeader(refreshingBlock: {
     self.freshList()
})

let footer = MJRefreshAutoNormalFooter(refreshingBlock: {
    self.loadMoreList()
})
footer?.triggerAutomaticallyRefreshPercent = -20

header.beginRefreshing()后，freshList()、loadMoreList()都调用了，定位到.reloadData()触发了footer的refresh

方法：
footer?.triggerAutomaticallyRefreshPercent = -20