[FIX] web_editor: update attribute of edition container

In most use case, when we edit the website we have a situation such as:

```
<div id="wrap" branding-attributes="...">
    <section class="mycontent">hi!</section>
</div>
```

When we modify a part, we replace all child nodes of the branded
element.

But if we had more complex content such as:

```
<div id="wrap">
    <section class="mycontent" branding-attributes="...">ho!</section>
    <t t-call-assets="web.assets_common" t-js="false" t-css="false"/>
</div>
```

we have a t-call inside the div#wrap, so branding is distributed to
child that could have attribute modified (eg. changing background).

Then if `<section/>` node is saved, the possibly modified attributes
are lost.

Without the change, the added test fails with:

 '<div class="nice">hoi</div>' not found in '...<div>hoi</div>...' :
 saved element attributes are saved excluding branding ones

opw-2122947
closes #40345

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>