codePlugin breaks code nested in other items

According to initial markdown syntax, following code is valid:

```markdown
*   bullet 1
    
        [php]
        <?php
            $a = null;
        ?>
    
*   bullet 2
```

NOTE: the line between `*   bullet 1` and `        [php]` contains exactly 4 spaces so that it continues the list item.

and should be translated as

```html
<ul>
<li><p>bullet 1</p>

<pre><code>[php]
&lt;?php
    $a = null;
?&gt;
</code></pre></li>
<li><p>bullet 2</p></li>
</ul>
```