Problem with new grid table

Consider:

```
<table>
<tr>
<td colspan="2">A</td>
</tr>
<tr>
<td>B</td>
<td>C</td>
</tr>
</table>
```

Result after markdown conversion:
```
+---+
| A |
+---+
| B | C |
+---+---+
```

Expected:
```
+-------+
| A     |
+---+---+
| B | C |
+---+---+
```

I suggest using the [planets.html](https://github.com/jgm/pandoc/blob/main/test/tables/planets.html5) as a general test.