Add onRawStatus to WebClient

- Add onRawStatus to WebClient.ResponseSpec, allowing users to deal with
  raw status codes that are not in HttpStatus.
- No longer throw an exception status codes not in HttpStatus.

Closes gh-23367

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -56,7 +56,11 @@ import org.springframework.http.client.reactive.JettyClientHttpConnector;
 import org.springframework.http.client.reactive.ReactorClientHttpConnector;
 import org.springframework.http.codec.Pojo;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
 
 /**
  * Integration tests using an {@link ExchangeFunction} through {@link WebClient}.
@@ -606,6 +610,28 @@ public class WebClientIntegrationTests {
 		});
 	}
 
+	@Test
+	public void shouldApplyCustomRawStatusHandler() {
+		prepareResponse(response -> response.setResponseCode(500)
+				.setHeader("Content-Type", "text/plain").setBody("Internal Server error"));
+
+		Mono<String> result = this.webClient.get()
+				.uri("/greeting?name=Spring")
+				.retrieve()
+				.onRawStatus(value -> value >= 500 && value < 600, response -> Mono.just(new MyException("500 error!")))
+				.bodyToMono(String.class);
+
+		StepVerifier.create(result)
+				.expectError(MyException.class)
+				.verify(Duration.ofSeconds(3));
+
+		expectRequestCount(1);
+		expectRequest(request -> {
+			assertEquals("*/*", request.getHeader(HttpHeaders.ACCEPT));
+			assertEquals("/greeting?name=Spring", request.getPath());
+		});
+	}
+
 	@Test
 	public void shouldApplyCustomStatusHandlerParameterizedTypeReference() {
 		prepareResponse(response -> response.setResponseCode(500)