[Bug Report] Search组件iOS端，选择键盘备选字触发两次@on-change

### VUX version
2.8.1

### OS/Browsers version
iOS/webkit

### Vue version
2.5.2

### Code

```html

```

### Steps to reproduce
iOS端，手机键盘输入关键字，点击要选择备选关键字时触发两次on-change事件

### What is Expected?
修复触发两次on-change的bug，谢谢

### What is actually happening?
修复触发两次on-change的bug，谢谢

Start by reproducing the provided Search component example on iOS/WebKit while selecting an IME suggestion. Trace the Search component's change handling and verify when the duplicate event occurs. Done means selecting one keyboard suggestion emits exactly one on-change event.

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