Thematic break and Front matter is not working in `to_html`

[luozhonghe]

When the to_html function converts the contents of a Thematic break or Front matter block in Markdown, unexpected HTML tags appear( <h2> or <p>).

Input yaml metadata block:

---
title: Markdown Front Matter
author: Bob
---

Output:

<h2>title: Markdown Front Matter
author: Bob</h2>

Input toml metadata block:

+++
title  = "Markdown Front Matter"
author = "Bob"
+++

Output:

<p>+++
title  = &quot;Markdown Front Matter&quot;
author = &quot;Bob&quot;
+++</p>

Input toml metadata block with yaml style:

---toml
title  = "Markdown Front Matter"
author = "Bob"
---

Output:

<h2>---toml
title  = &quot;Markdown Front Matter&quot;
author = &quot;Bob&quot;</h2>