Spring BufferClosedException when decoding to Flux in StringDecoder

Following the upgrade to Netty 5 in #28847, there is one remaining WebFlux test that fails.

The test case is a 500 error, suppressed with a status handler, so the response body is decoded as normal. However, it appears to not be related to the suppressed status. It can be reproduced with a normal response decoded to Flux<String>.

// WebClientIntegrationTests

@Test
void retrieveTextDecodedToFlux() {

    ReactorNetty2ClientHttpConnector connector = new ReactorNetty2ClientHttpConnector();
    startServer(connector);

    prepareResponse(response -> response
            .setHeader("Content-Type", "text/plain")
            .setBody("Hey now"));

    Flux<String> result = this.webClient.get()
            .uri("/")
            .accept(MediaType.TEXT_PLAIN)
            .retrieve()
            .bodyToFlux(String.class);

    StepVerifier.create(result)
            .expectNext("Hey now")
            .expectComplete()
            .verify(Duration.ofSeconds(3));
}

StringDecoder has a more advanced buffer manipulation when decoding for Flux. This is most likely related to the changes in #28874.