在el-table表头添加icon图标，鼠标经过显示el-tooltip提示内容

<el-table-column prop="date" label="操作" align="center">
  <template slot="header">
<span>操作</span>
<el-tooltip popper-class="tooltip" placement="top">
  <i class="el-icon-warning-outline"></i>
  <div slot="content" class="tooltip-content">
<div>标题提示内容</div>
  </div>
</el-tooltip>
  </template>
  <template slot-scope="scope">
<el-button
  size="mini"
  style="padding: 5px 7px"
  type="success"
  icon="el-icon-document"
  @click="btn(scope.$index, scope.row)"
  >按钮</el-button
>
  </template>
</el-table-column>