helpers with style of array(Object, methodName)

Previously we had them working. So for the sake's of backward
compatibility, they should be supported.

src/Handlebars/Helpers.php

@@ -129,7 +129,7 @@ class Helpers
             );
         }
 
-        return $this->helpers[$name]($template, $context, $args, $source);
+        return call_user_func($this->helpers[$name], $template, $context, $args, $source);
     }
 
     /**