fix tasklist XHTML render

[acmepjz]

Currently the XHTML render of the following markdown

- [ ] Is this valid XHTML?
- [x] Is this valid XHTML?

is

<ul>
<li class="task-list-item"><input type="checkbox" class="task-list-item-checkbox" disabled>Is this valid XHTML?</li>
<li class="task-list-item"><input type="checkbox" class="task-list-item-checkbox" disabled checked>Is this valid XHTML?</li>
</ul>

which is not a valid XHTML. This PR adds check code to it which makes them valid XHTML.