adding a test for html doctype embeded in a <pre><code>

When an item contains: 

    <pre><code>
    #!html
    &lt;!DOCTYPE html>
    </code></pre>

with the relativize_paths filter, the result is messed up by the [following code](https://github.com/nanoc/nanoc/blob/release-3.6.x/lib/nanoc/filters/relativize_paths.rb#L98):

    # FIXME cleanup because it is ugly
    # # Because using the `Nokogiri::XML::DocumentFragment` class DOCTYPE 
    # pseudonodes becomes even more creepy than usual.
    result.sub!(/(!DOCTYPE.+?)(&gt;)/, '<\1>')

Is that still usefull?