HTML export: Export src block language as css class

Org mode exports source blocks with the language as a css-class of the exported
source block - it's easy enough to do the same.

org/testdata/headlines.html

@@ -68,9 +68,11 @@ headline with custom status
 <p>
 it&#39;s possible to use <code class="verbatim">#+SETUPFILE</code> - in this case the setup file contains the following
 </p>
+<pre class="src src-org">
 <div class="highlight">
 <pre>
 #+TODO: TODO DONE CUSTOM
 #+EXCLUDE_TAGS: noexport custom_noexport
 </pre>
 </div>
+</pre>