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 have no *strict* religion about fat vs slim tests. :smiley: 
In cases like this one, I think having only one "evolutive" test makes it somehow easier to follow, but that's only my opinion as well.
I agree about the docstring though!