Fix parsing table cells (#3904)
This PR:
- Fixes removing HTML tags that exist in <td> cells
- stripping function was in general problematic to implement in easy and
straightforward way (you can't modify `descendants` in-place). So I
decided instead of patching something in table cell I added stripping
everywhere in the same consistent way. This is why some tests needed
small edits with removing one white-space in each tag. I believe this
won't cause any problems for downstream tasks.
Tested HTML:
```html
<table class="Table">
<tbody>
<tr>
<td colspan="2">
Some text
</td>
<td>
<input checked="" class="Checkbox" type="checkbox"/>
</td>
</tr>
</tbody>
</table>
```
Before & After
```html
'<table class="Table" id="..."> <tbody> <tr> <td colspan="2">Some text</td><td></td></tr></tbody></table>'
'<table class="Table" id="..."><tbody><tr><td colspan="2">Some text</td><td><input checked="" type="checkbox"/></td></tr></tbody></table>''
```
