Allow live node preview access to user that has 'create' OR 'update' permission

If a user has only one of the permissions: 'create NODE TYPE', 'update NODE TYPE', but not both, access is denied on live preview.
The attached patch changes this behavior and allows access either permission is present.
This is not an optimal solution: the best thing would be to load the form from cache and check if we are creating or editing the node, then apply the corresponding permission check.