h2m shouldn't convert some <pre> blocks to indented blocks

[teoli2003]

When converting HTTP to Markdown in mdn/content#7853, @hamishwillee noticed that the converter was transforming <pre> blocks without any brush applied to this format:

    Pre block content

while we prefer:

 ```
 Pre block content
 ```

This may also happen for <pre class="brush: plain">.

We have quite a few occurrences of these, so it is likely worth fixing the converter.