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/keywords.html

@@ -1,8 +1,9 @@
 <figure>
-<div class="highlight a b c d" id="it">
+<pre class="src src-sh a b c d" id="it"><div class="highlight">
 <pre>echo &#34;a bash source block with custom html attributes&#34;
 </pre>
 </div>
+</pre>
 <figcaption>
 and <span style="text-decoration: underline;">multiple</span> lines of <strong>captions</strong>!
 </figcaption>