[Bug Report] loadMore在嵌套容器内 会直接引发上拉刷新方法

### Mint UI version
2.2.7

### OS/Browsers version
Win7/谷歌58 Iphone7 都有这问题

### Vue version
2.3.4

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

### Steps to reproduce
我直接使用你们官网的例子吧。
把区域拉到底部，就是看见 20 数字的时候，然后往上拉动，并不是实际的向上滑动 而是触发了上拉加载的方法

### What is Expected?
我想要的是正常的往上滑动，滑到顶部才触发它的上拉加载

### What is actually happening?
实际行为是只要往上拉，就会触发上拉加载

Reproduce the issue using the pull-down example at http://elemefe.github.io/mint-ui/#/pull-down with Mint UI 2.2.7 and Vue 2.3.4. Inspect the nested container and loadMore behavior, then verify that upward scrolling only triggers loading at the intended boundary rather than during normal movement.

Written by the indexing model from the issue text.