HTML entities are not encoded when using plain text

[svivian]

Using this Markdown code:

```text
<b>test</b>
```

The HTML is not encoded and I end up with a code block that just says "test", in bold. In other words this is the HTML output:

<pre class="language-text"><code class="language-text"><b>test</b></code></pre>

It should be this:

<pre class="language-text"><code class="language-text">&lt;b&gt;test&lt;/b&gt;</code></pre>

Using any other language (e.g. js) encodes the entities correctly. As does omitting a language (just using ```) but then you don't get the default highlighting (box colour etc).

[zachleat]

Duplicate of #54 (comment) (some discussion and solutions over there, thank you!)