Pay submit handlers unnecessarily destroy $form_state['storage']

In the Webform Pay issue queue, we've found a way to have failed transactions send the user right back to the page they were on without losing form information. So on a multiple page form, the user is sent back to the last page with all their information intact (except the Pay field, which clears itself).

However in Webform we keep track of which page the user is on and all the current values in $form_state['storage']. If the transaction fails, we need that information to still be there so the user can correct the mistake and submit the form again. In Pay's submit() method's though, $form_state['storage'] is entirely unset(). I can't find a reason for this however, as Pay doesn't actually put anything into storage so it shouldn't be removing it.

Even if Pay *did* put something into storage, a safer approach would be putting everything into $form_state['storage']['pay'] = array(), then just unsetting the single part of the storage that Pay inserted into storage. Then other modules (like Webform) can manage their own storage without Pay removing everything.

For the time being though, I've simply removed the calls to unset() entirely, since $form_state['storage'] isn't used in any way, there's no need to unset() it.