HTML tags that are `display:block` by default are messed up when multiline

Certain HTML tags when messed up when multiline, and as far as I can tell it's the ones that are display:block by default.

Found on v2.3.6, installed from the FreeBSD py37-markdown2 port; I checked out the repo and ran v2.3.8 to make sure and it was not fixed.

Testcases (I use <wc> because I originally ran into this with a custom web component):

---

<wc>
<div>
</div>
</wc>

Produces (incorrect):

<p><wc></p>

<div>
</div>

<p></wc></p>

---

<wc>  
<pre>
</pre>
</wc>

Produces (incorrect):

<p><wc></p>

<pre>
</pre>

<p></wc></p>

---

<wc>
<h1>
</h1>
</wc>

Produces (incorrect):

<p><wc></p>

<h1>
</h1>

<p></wc></p>

---

<wc>
<span>
</span>
</wc>

Produces (correct):

<p><wc>
<span>
</span>
</wc></p>

---

<wc>
<invalid>
</invalid>
</wc>

Produces (correct):

<p><wc>
<invalid>
</invalid>
</wc></p>

Start by reproducing the listed custom-element cases with the repository's markdown2 entry point, then trace how multiline HTML tags are classified and paragraph-wrapped. Done means block-level tags such as div, pre, and h1 remain inside the surrounding wc element, while span and invalid-tag behavior stays correct; add regression coverage for these examples.