system_mail() should get the node form $context not $params

But the node was being looked for in the wrong place
Function system_mail
line 1727 & 1728 currently read
if (isset($params['node'])) {
$node = $params['node'];

should read
if (isset($context['node'])) {
$node = $context['node'];

This means that when the workflow module sends a mail, the node variables such as %node_url are not replaced