The exchange() method is now deprecated because it is not safe for
general use but that doesn't apply to the WebTestClient because it
exposes a different higher level API for response handling that
ensures the response is consumed. Nevertheless WebTestClient cannot
call WebClient.exchange() any more.
To fix this WebTestClient no longer delegates to WebClient and thus
gains direct access to the underlying ExchangeFunction. This is
not a big deal because WebClient and WebTestClient do the same
only when it comes to gathering builder options and request input.
See gh-25751
db9190e0e6