Deep copy `token.attrs` to avoid mutating token stream

lib/renderer.js

@@ -64,7 +64,7 @@ default_rules.fence = function (tokens, idx, options, env, slf) {
   // now we prefer to keep things local.
   if (info) {
     i        = token.attrIndex('class');
-    tmpAttrs = token.attrs ? token.attrs.slice() : [];
+    tmpAttrs = token.attrs ? JSON.parse(JSON.stringify(token.attrs)) : [];
 
     if (i < 0) {
       tmpAttrs.push([ 'class', options.langPrefix + langName ]);