Put class name on <pre>, rather than <code>

Better CSS styling options ensue, if we put
the class='lang' on the <pre> element.

(Suggested by Casper Gripenberg)

vendor/plugins/maruku/lib/maruku/output/to_html.rb

@@ -574,10 +574,10 @@ and
 				
 				code = Document.new(html, {:respect_whitespace =>:all}).root
 				code.name = 'code'
-				code.attributes['class'] = lang
 				code.attributes['lang'] = lang
-				
+							
 				pre = Element.new 'pre'
+				pre.attributes['class'] = lang
 				pre << code
 				pre
 			rescue LoadError => e