Add a hook_pay_method_alter

I was trying to add some customizations to the a webform_pay form to meet my client's business requirements (default the expiration date selects to blank, show the required asterisk on these fields), but I couldn't do this with a standard hook_form_alter because webform_pay (and maybe pay) do a lot of the form attachment in pre_render functions.

Adding a drupal_alter to create a hook_pay_method_alter solved it for me. The applied patch adds the drupal_alter to pay_method_gateway.inc. If you are willing to commit this, I'll add this to the other pay_method classes and pay.api.php.