header插槽，作用域传入selected数组

```vue
<template>
  <el-data-table
    v-bind="$data"
  >
    <template v-slot:header="{selected}">
      <el-tag>slot=header{{selected.length}}</el-tag>
      <el-dropdown>
        <el-button type="primary" size="small">
          更多菜单<i class="el-icon-arrow-down el-icon--right"></i>
        </el-button>
        <el-dropdown-menu slot="dropdown">
          <el-dropdown-item>黄金糕</el-dropdown-item>
          <el-dropdown-item>狮子头</el-dropdown-item>
          <el-dropdown-item>螺蛳粉</el-dropdown-item>
          <el-dropdown-item>双皮奶</el-dropdown-item>
          <el-dropdown-item>蚵仔煎</el-dropdown-item>
        </el-dropdown-menu>
      </el-dropdown>
    </template>
  </el-data-table>
</template>
<script>
export default {
  data() {
    return {
      url: 'https://mockapi.eolinker.com/IeZWjzy87c204a1f7030b2a17b00f3776ce0a07a5030a1b/el-data-table?q=basic',
      columns: [
        {prop: 'date', label: '日期'},
        {prop: 'name', label: '姓名'},
        {prop: 'address', label: '地址'},
      ],
    }
  }
}
</script>
```
