fence renderer: fix concat of class array

Before:

  <pre c="l"><code>...

Now:

  <pre class="someClass language-python><code>...

Bug introduced in b7c868b64b

lib/renderer.js

@@ -68,7 +68,7 @@ default_rules.fence = function (tokens, idx, options, env, slf) {
     if (i < 0) {
       tmpAttrs.push([ 'class', options.langPrefix + langName ]);
     } else {
-      tmpAttrs[i] += ' ' + options.langPrefix + langName;
+      tmpAttrs[i][1] += ' ' + options.langPrefix + langName;
     }
 
     // Fake token just to render attributes