Fixed charge, credit, and refund methods to pass data as an XML payload.
Fixed charge, credit, and refund methods to pass data as an XML payload.

Before this changeset, Active Resource was passing the subscription as
the payload and passing the parameters via the URL.

chase439/chargify_api_ares,elfotografo007/chargify_api_ares,carsonreinke/chargify_api_ares,outstand/chargify_api_ares,wsmoak/chargify_api_ares,chargify/chargify_api_ares