def render_string(template_string, context, format = default_format)
      render_file(proc_template(template_string), context, format)
    end
def render_string(template_string, context, format = default_format)
      render_file(proc_template(template_string), context, format)
    end
need an explicit #render_string method so it's possible to distinguish
 between a String which is a path to a template & a String which is a
 template itself.