Fake all methods required by cancellation button

For [this bug](https://github.com/netguru/spree_payu_integration/issues/5) to be solved, we will also need methods as in this PR.

Tested on our staging.
Unused method argument - `response`. If it's necessary, use `_` or `_response` as an argument name to indicate that it won't be used. You can also write as `cancel(*)` if you want the method to accept any arguments but don't care about them.