view.DowncastWriter does not handle partial unwrapping in nested attribute elements

[scofalik]

For given view and selection:

<p>
    [<i>
        <span class="foo bar">Foo</span>
    </i>]
</p>

Unwrapping <span class="foo"> will not do anything.

DowncastWriter#unwrap() is incorrectly implemented in this regard. When it enters a child, it only looks for full matches, there's no partial unwrapping there.

EDIT: AFAICS the selection doesn't need to be exactly on the <i> element, it can be somewhere inside the <span>.

[scofalik]

Also -- will have to check if wrapping works correctly.

[scofalik]

Wrapping is incorrect too.