[Bug Report] 【Demo的bug】loadmore上拉加载更多在iOS微信上异常

### Mint UI version
2.2.7

### OS/Browsers version
iOS微信 微信版本6.5.9 iOS版本 10.3.2

### Vue version
2.3.4

### Reproduction Link
[http://elemefe.github.io/mint-ui/#/pull-up](http://elemefe.github.io/mint-ui/#/pull-up)

### Steps to reproduce
1，打开重现连接（即官网首页的二维码项目地址pull-up的页面）
2，手指按住往上滑动，或者快速滚动到底部再往上拉
3，很大的几率会出现 上拉箭头未显示、未进入loading状态。

### What is Expected?
正常的上拉加载

### What is actually happening?
很大的几率出现无法上拉加载
具体的表现为 上拉箭头未显示，无法上拉、未进入loading状态

Start with the pull-up reproduction link and reproduce the failure in iOS WeChat 6.5.9 on iOS 10.3.2 using Vue 2.3.4 and Mint UI 2.2.7. Trace the pull-up component from that demo to determine why the arrow and loading state are skipped; done means upward loading reliably enters its loading state.

Written by the indexing model from the issue text.