Fix issue with refunds

| Q             | A
| ------------- | ---
| Bug fix?      |  yes
| New feature?  |  no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

This PR tries to solve an issue we've been experiencing this week with the refunds. We don't know the reason but when making refunds with production enviroment it always throws a PaymillException. The funny part is that the expection has a 20000 response code.

We've been debugging the comunication between paymill api and our systems and we've realized that the response code was casted to string instead of integer. We've changed the strict type comparision in response handler.