Content negotiation for exception handling fails with WebFlux

[bclozel]

We have found a bug while running the Spring Boot test suite against the latest Spring Framework milestone.

The new content negotiation for error handling feature added in #31936 fails in WebFlux when HTTP clients send invalid media types in the "Accept" header. Instead of falling back on "*/*" as it should, the error handling selection fails on parsing the requested media types and a NotAcceptableStatusException is thrown.