make fb_connect_async_js return a html_safe string for rails 3

lib/facebooker2/rails/helpers/javascript.rb

@@ -32,9 +32,13 @@ module Facebooker2
             }());
           </script>
           JAVASCRIPT
+          if ::Rails::VERSION::STRING > "2"
+            js.html_safe
+          else
             block_given? ? concat(js) : js
           end
         end
       end
     end
+  end
 end