CSS and JS always empty in the object returned by fl-builder/v1/123

[TBarregren]

The REST API call fl-builder/v1/123 returns an object with the properties id, html, css and js. The two latter are always empty strings. Following implementation of FL_Rest_Content::callback() works as expected:

static function callback( $request ) {

  $post_id = FL_Rest::get_id( $request );

  ob_start();
  \FLBuilder::render_content_by_id( $post_id );
  $output->html = ob_get_clean();

  \FLBuilderModel::set_post_id( $post_id );
  $output->css = FLBuilder::render_css(false);
  $output->js = FLBuilder::render_js(false);
  \FLBuilderModel::reset_post_id();

}