Html renderer: don't add language- to code block class...

if the info string already starts with `language-`.
Closes #277.

lib/render/html.js

@@ -144,7 +144,11 @@ function code_block(node) {
     var info_words = node.info ? node.info.split(/\s+/) : [],
         attrs = this.attrs(node);
     if (info_words.length > 0 && info_words[0].length > 0) {
-        attrs.push(["class", "language-" + this.esc(info_words[0])]);
+        var cls = this.esc(info_words[0]);
+        if (!/^language-/.exec(cls)) {
+          cls = "language-" + cls;
+        }
+        attrs.push(["class", cls]);
     }
     this.cr();
     this.tag("pre");

test/regression.txt

@@ -455,3 +455,19 @@ _*xx-_-
 <p><em>*__-</em>-</p>
 <p><em>*xx-</em>-</p>
 ````````````````````````````````
+
+#277:
+```````````````````````````````` example
+```language-r
+x <- 1
+```
+
+```r
+x <- 1
+```
+.
+<pre><code class="language-r">x &lt;- 1
+</code></pre>
+<pre><code class="language-r">x &lt;- 1
+</code></pre>
+````````````````````````````````