cell-default-arrow-width设置箭头width后不居中

### VUX version
2.9.0

### OS/Browsers version
any

### Vue version
2.5.17-beta.0


### Code

``css
@cell-default-arrow-width: 16px;

//weui-access.less 第 9 行 .weui-cell__ft 的代码，这里有问题：

.weui-cell__ft {
padding-right: 13px;
position: relative;
&:after {
content: " ";
.setArrow(right, @cell-default-arrow-width, @cell-default-arrow-color, @cell-default-arrow-border-width);
position: absolute;
top: 50%;
margin-top: -4px;
right: 2px;
}
}

//------------------------------------------------------
//既然箭头可以通过 @cell-default-arrow-width 设置尺寸， 那top 和margin-top 就不应该写死。
```

### Steps to reproduce
设置 cell-default-arrow-width 设置箭头大小。

### What is Expected?
设置 cell-default-arrow-width 后，箭头还能保持上下居中

### What is actually happening?
设置 cell-default-arrow-width 后，箭头不能上下居中