Regression: WebClient with unknown response code results in IllegalArgumentException

[blake-bauman]

It appears that the issue reported in #1393 has come back in some scenarios. This was fixed in 2.1.3, but seems to be back in 2.1.4. It worked with my original test, so I didn't notice it right away, but it seemed someone found where it's no longer working.

I've modified the sample slightly from the previous issue to reproduce the problem every time: https://github.com/blake-bauman/test-sleuth-unknown-status-code

When I tested it earlier, it appeared to be working with 2.1.4, but didn't notice that the server was still throwing the same exception, even though the request was "successful". A slight change to the code (https://github.com/blake-bauman/test-sleuth-unknown-status-code/commit/e572b48e47a8d2cc48c210660c1dd1bbfe0b4cdb) gets the request itself to fail, the same as before. Both scenarios were working properly in 2.1.3.

Exception is still the same:

java.lang.IllegalArgumentException: No matching constant for [499]
	at org.springframework.http.HttpStatus.valueOf(HttpStatus.java:532) ~[spring-web-5.1.10.RELEASE.jar:5.1.10.RELEASE]
	at org.springframework.http.client.reactive.ReactorClientHttpResponse.getStatusCode(ReactorClientHttpResponse.java:90) ~[spring-web-5.1.10.RELEASE.jar:5.1.10.RELEASE]
	at org.springframework.web.reactive.function.client.DefaultClientResponse.statusCode(DefaultClientResponse.java:75) ~[spring-webflux-5.1.10.RELEASE.jar:5.1.10.RELEASE]
	at org.springframework.cloud.sleuth.instrument.web.client.TraceExchangeFilterFunction$MonoWebClientTrace$WebClientTracerSubscriber.terminateSpan(TraceWebClientBeanPostProcessor.java:349) ~[spring-cloud-sleuth-core-2.1.4.RELEASE.jar:2.1.4.RELEASE]
	at org.springframework.cloud.sleuth.instrument.web.client.TraceExchangeFilterFunction$MonoWebClientTrace$WebClientTracerSubscriber.onNext(TraceWebClientBeanPostProcessor.java:300) ~[spring-cloud-sleuth-core-2.1.4.RELEASE.jar:2.1.4.RELEASE]
	at org.springframework.cloud.sleuth.instrument.web.client.TraceExchangeFilterFunction$MonoWebClientTrace$WebClientTracerSubscriber.onNext(TraceWebClientBeanPostProcessor.java:232) ~[spring-cloud-sleuth-core-2.1.4.RELEASE.jar:2.1.4.RELEASE]
	at org.springframework.cloud.sleuth.instrument.reactor.ScopePassingSpanSubscriber.onNext(ScopePassingSpanSubscriber.java:96) ~[spring-cloud-sleuth-core-2.1.4.RELEASE.jar:2.1.4.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:114) ~[reactor-core-3.2.12.RELEASE.jar:3.2.12.RELEASE]
	at org.springframework.cloud.sleuth.instrument.reactor.ScopePassingSpanSubscriber.onNext(ScopePassingSpanSubscriber.java:96) ~[spring-cloud-sleuth-core-2.1.4.RELEASE.jar:2.1.4.RELEASE]