fix up generator normaliseOptions

lib/6to5/generation/generator.js

@@ -35,21 +35,17 @@ _.each(Buffer.prototype, function (fn, key) {
 });
 
 CodeGenerator.normaliseOptions = function (opts) {
-  opts = opts.format || {};
-
-  opts = _.merge({
+  return _.merge({
     parentheses: true,
     semicolons: true,
-    comments: opts.comments,
+    comments: opts.comments == null || opts.comments,
     compact: false,
     indent: {
       adjustMultilineComment: true,
       style: "  ",
       base: 0
     }
-  }, opts);
-
-  return opts;
+  }, opts.format || {});
 };
 
 CodeGenerator.generators = {