Added quotes around language class attribute on github style codeblock

src/showdown.js

@@ -915,7 +915,7 @@ var _DoGithubCodeBlocks = function(text) {
 			codeblock = codeblock.replace(/^\n+/g,""); // trim leading newlines
 			codeblock = codeblock.replace(/\n+$/g,""); // trim trailing whitespace
 
-			codeblock = "<pre><code" + (language ? " class=" + language : "") + ">" + codeblock + "\n</code></pre>";
+			codeblock = "<pre><code" + (language ? " class=\"" + language : "\"") + ">" + codeblock + "\n</code></pre>";
 
 			return hashBlock(codeblock);
 		}