Magento form_key validation failed in 1.8 after upgrade from 1.7

config.xml

            <controller_action_predispatch_checkout_cart_add>
                <observers>
                    <mymodul_fromkey>
                        <type>singleton</type>
                        <class>mymodul/observer</class>
                        <method>addDisableValidatFromKey</method>
                    </mymodul_fromkey>
                </observers>
            </controller_action_predispatch_checkout_cart_add>

Observer.php

<?php

class MyCompany_MyModul_Model_Observer
{
   /**
     * 
     *
     */
    public function addDisableValidatFromKey(Varien_Event_Observer $observer)
    {
        $controller = $observer->getControllerAction();
        $formKey = $controller->getRequest()->getParam('form_key', null);
        if(!is_null($formKey) && $formKey != Mage::getSingleton('core/session')->getFormKey())
        {
            $controller->getRequest()->setParam('form_key', Mage::getSingleton('core/session')->getFormKey());
        }
    }
}