Fix regression due to using request.REQUEST
Fix regression due to using request.REQUEST

At some point in the stack, we need to pass the query parameters around
and they end up being serialized as JSON. Using request.REQUEST,
however, leaves us with a non-serializable `MergedDict` object, and
everything goes to hell from that point. Let's not use request.REQUEST
any more.

django-oscar/django-oscar-adyen,oscaro/django-oscar-adyen