Fix HTML fenced code siblings management

[xavierlacot]

Because the commonmark HTML parsing algorithm is what it is, some cases can produce a strange AST.

For example,

<p>Intro</p>
<pre><code>Multiline

code
</code></pre>

will be parsed as:

<document xmlns="http://commonmark.org/xml/1.0">
  <html_block>&lt;p&gt;Intro&lt;/p&gt;
&lt;pre&gt;&lt;code&gt;Multiline</html_block>
  <paragraph>
    <text>code</text>
    <softbreak />
    <html_inline>&lt;/code&gt;</html_inline>
    <html_inline>&lt;/pre&gt;</html_inline>
  </paragraph>
</document>

while (note the extra line between the first two HTML nodes)

<p>Intro</p>

<pre><code>Multiline

code
</code></pre>

will be parsed as:

<document xmlns="http://commonmark.org/xml/1.0">
  <html_block>&lt;p&gt;Intro&lt;/p&gt;</html_block>
  <html_block>&lt;pre&gt;&lt;code&gt;Multiline

code
&lt;/code&gt;&lt;/pre&gt;</html_block>
</document>

This is perfectly in line with the specs of Commonmark, but was leading to situations where the HtmlBlockFixer could not correctly transform the input HTML into markdown and was generating errors. This PR fixes such a behavior.