/**
  * Método que genera el formulario de pago en HTML
  *
  * @param string $button_type
  *   Dimensión del boton a mostrar
  *
  * @return string
  *   Formulario renderizado
  */
 public function renderForm($button_type = '100x50')
 {
     $values = $this->getFormLabels();
     $html = new DOMDocument();
     $html->formatOutput = true;
     $form = $html->createElement('form');
     $form->setAttribute('action', $this->getApiUrl());
     $form->setAttribute('method', 'POST');
     foreach ($values as $name => $value) {
         $input_hidden = $html->createElement('input');
         $input_hidden->setAttribute('type', 'hidden');
         $input_hidden->setAttribute('name', $name);
         $input_hidden->setAttribute('value', $value);
         $form->appendChild($input_hidden);
     }
     $input_hidden = $html->createElement('input');
     $input_hidden->setAttribute('type', 'hidden');
     $input_hidden->setAttribute('name', 'agent');
     $input_hidden->setAttribute('value', $this->agent);
     $form->appendChild($input_hidden);
     $buttons = Khipu::getButtonsKhipu();
     if (isset($buttons[$button_type])) {
         $button = $buttons[$button_type];
     } else {
         $button = $buttons['100x50'];
     }
     $submit = $html->createElement('input');
     $submit->setAttribute('type', 'image');
     $submit->setAttribute('src', $button);
     $form->appendChild($submit);
     $html->appendChild($form);
     return $html->saveHTML();
 }