当a-select或者a-input-number 组件 绑定了 v-model 时，placeholder属性不起作用

- [ ] I have searched the [issues](https://github.com/vueComponent/ant-design/issues) of this repository and believe that this is not a duplicate.

### Version
 "vue-antd-ui": "^0.5.4"

### Environment
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### Reproduction link
[example](https://codesandbox.io/s/2wpk21kzvr)

### Steps to reproduce
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### What is expected?
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### What is actually happening?
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