A table without a header will fail to convert

For Example：
```
<table>
    <thead>
        <tr>
            <th></th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>content</td>
        </tr>
    </tbody>
</table>
```

will to be：
```
| content |
```

I hope this:
```
|  |
|--- |
| content |
```