Table parsing pull previous content into the header

Input:
```
echo (new Parsedown)->text(<<<MD
A

| B | C |
|-------------|---------|
| B1 | C1 |
MD);
```

Output:
```
<table>
<thead>
<tr>
<th>A</th>
<th>B</th>
<th>C</th>
</tr>
</thead>
<tbody>
<tr>
<td>B1</td>
<td>C1</td>
</tr>
</tbody>
</table>
```

Note that A has become a table header instead of being a separate paragraph.