[12.0][FIX] action_cancel resets the workflow

If someone cancel the invoice reset the "automated workflow" to null to make sure it is not processed anymore.

sale_automatic_workflow/models/account_invoice.py

@@ -13,3 +13,10 @@ class AccountInvoice(models.Model):
         comodel_name='sale.workflow.process',
         string='Sale Workflow Process'
     )
+
+    @api.multi
+    def action_cancel(self):
+        for inv in self:
+            if inv.workflow_process_id:
+                inv.workflow_process_id = None
+        return super(AccountInvoice, self).action_cancel()