WebFlux returns 406 but Spring MVC renders "text/plain" request based on Publisher<?> and String [SPR-17078]

Dave Syer opened SPR-17078 and commented

I assume this is a Framework issue (might be Spring Boot I guess, since that's what I use to test it).

Controller:

@GetMapping("/straight")
public Mono<ResponseEntity<Publisher<?>>> straight() {
     return Mono.just(ResponseEntity.ok().header("x-foo", "spam")
               .body(Mono.from(Flux.just("foo", "bar"))));
}

Test:

@Test
public void straight() throws Exception {
     ResponseEntity<String> result = rest.exchange(RequestEntity
               .get(new URI("/straight")).accept(MediaType.TEXT_PLAIN).build(),
               String.class);
     assertThat(result.getBody()).isEqualTo("foo");
     assertThat(result.getHeaders()).containsKey("x-foo");
}

The test passes with MVC and fails with Webflux (406 not acceptable).

Affects: 5.0.7

Start by reproducing the supplied /straight controller and straight test on Spring MVC and WebFlux with version 5.0.7, focusing on the ResponseEntity containing a Publisher<?> and the text/plain request. Trace the WebFlux response handling after the 406 occurs. Done means the WebFlux case matches the MVC result, including the foo body and x-foo header.