Fix origin IP address retrieval

This PR allows one to configure the name of the HTTP header in which the IP address from which a payment originated can be found.

The canonical default of `'REMOTE_ADDR'` is assumed in case there is no `ADYEN_IP_ADDRESS_HTTP_HEADER` setting specified in the Django project — which should be the case if any component of the HTTP stack transfers this information in another HTTP header (often, but not always `'HTTP_X_FORWARDED_FOR'`).
I don't get it.

I mean: if it is a unit-test, you should definitively assert that something have been modified somewhere, somehow. If you can't, IMHO, it means either it is not a unit-test, or there is an issue with testability of this component.

Not being a unit-test is not an issue but it should be in a tests/integration/your_test.py file for easy filtering.