In development: the request's address, then the last registered one, then
the built-in mock's. In product: only a registered address; a request
naming any other is refused, and there is no fallback.
Parameters
spec: ReturnAddressSpec
requested, registered, unconfirmed, fallback, and the
attribute, parameter and application a refusal names
Returns ReturnAddressAnswer
{ ok, url, from }, or { ok: false, why } carrying its error
code
Resolves the address a response is delivered to.
In development: the request's address, then the last registered one, then the built-in mock's. In product: only a registered address; a request naming any other is refused, and there is no fallback.