markdown_github does not generate code blocks with backticks

[pedrolamas]

If I create a file with the following HTML

<pre class="csharp"><code>teste();</code></pre>

And then use pandoc with this command line:

pandoc --from html --to markdown_github test.htm

This is the output

~ ~ ~ ~ {.csharp}
teste();
~ ~ ~ ~

not even if I use --to markdown+backtick_code_blocks seems to do the job properly