Disable "failOnServerError" in ReactorClientHttpRequest

This commit disables the "failOnServerError" feature on the
`HttpClientRequest`, as wrapped by ReactorClientHttpRequest. 5xx errors
are supposed to be dealt with in the WebClient, not in the lower-level
components.

Issue: SPR-15739

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -53,7 +53,7 @@ public class ReactorClientHttpRequest extends AbstractClientHttpRequest {
 			HttpClientRequest httpRequest) {
 		this.httpMethod = httpMethod;
 		this.uri = uri;
-		this.httpRequest = httpRequest.failOnClientError(false);
+		this.httpRequest = httpRequest.failOnClientError(false).failOnServerError(false);
 		this.bufferFactory = new NettyDataBufferFactory(httpRequest.alloc());
 	}
 

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

@@ -370,6 +370,26 @@ public class WebClientIntegrationTests {
 		Assert.assertEquals("/greeting?name=Spring", recordedRequest.getPath());
 	}
 
+	@Test
+	public void retrieveBodyToMonoInternalServerError() throws Exception {
+		this.server.enqueue(new MockResponse().setResponseCode(500)
+				.setHeader("Content-Type", "text/plain").setBody("Internal Server error"));
+
+		Mono<String> result = this.webClient.get()
+				.uri("/greeting?name=Spring")
+				.retrieve()
+				.bodyToMono(String.class);
+
+		StepVerifier.create(result)
+				.expectError(WebClientException.class)
+				.verify(Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		Assert.assertEquals(1, server.getRequestCount());
+		Assert.assertEquals("*/*", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+		Assert.assertEquals("/greeting?name=Spring", recordedRequest.getPath());
+	}
+
 	@Test
 	public void retrieveToEntityNotFound() throws Exception {
 		this.server.enqueue(new MockResponse().setResponseCode(404)