[Bug Report] checker的样式default-item-class、selected-item-class用$style.时不能注入

### VUX version
2.9.2

### OS/Browsers version
win10，chrome 69.0.3497.81

### Vue version
2.5.17

### Code

```html

.defaultCheckerItem {
display: block;
background-color: #f3f4f6;
border: 1px solid #ececec;
border-radius: 4px;
margin: 15px 21px 0 21px;
height: 27px;
}
.selectedCheckerItem {
border: 1px solid #3b99fc;
}

```

### Steps to reproduce
vue小白，不知道这个能不能算是bug，只是遇到这个问题，不知道该怎么解决，如果有时间请不吝赐教

### What is Expected?

### What is actually happening?

Start with the checker component and reproduce the provided Vue 2.5.17 example using default-item-class and selected-item-class with CSS modules. Compare the rendered classes with the expected output, then verify that both props preserve the $style-resolved class names.

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