Support custom status code in ExchangeResult for WebTestClient

Prior to this commit, ExchangeResult.assertWithDiagnostics() threw an
IllegalArgumentException for a custom HTTP status code since toString()
invoked getStatus() without a try-catch block.

This commit addresses this issue by introducing a formatStatus() method
that defensively formats the response status, initially trying to
format the HttpStatus and falling back to formatting the raw integer
status code.

Closes gh-29283

spring-test/src/main/java/org/springframework/test/web/reactive/server/ExchangeResult.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -50,6 +50,7 @@ import org.springframework.util.MultiValueMap;
  * respectively.
  *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  * @since 5.0
  * @see EntityExchangeResult
  * @see FluxExchangeResult
@@ -171,6 +172,8 @@ public class ExchangeResult {
 
 	/**
 	 * Return the HTTP status code as an {@link HttpStatus} enum value.
+	 * @throws IllegalArgumentException in case of an unknown HTTP status code
+	 * @see #getRawStatusCode()
 	 */
 	public HttpStatus getStatus() {
 		return this.response.getStatusCode();
@@ -248,13 +251,22 @@ public class ExchangeResult {
 				"\n" +
 				formatBody(getRequestHeaders().getContentType(), this.requestBody) + "\n" +
 				"\n" +
-				"< " + getStatus() + " " + getStatus().getReasonPhrase() + "\n" +
+				"< " + formatStatus() + "\n" +
 				"< " + formatHeaders(getResponseHeaders(), "\n< ") + "\n" +
 				"\n" +
 				formatBody(getResponseHeaders().getContentType(), this.responseBody) +"\n" +
 				formatMockServerResult();
 	}
 
+	private String formatStatus() {
+		try {
+			return getStatus() + " " + getStatus().getReasonPhrase();
+		}
+		catch (Exception ex) {
+			return Integer.toString(getRawStatusCode());
+		}
+	}
+
 	private String formatHeaders(HttpHeaders headers, String delimiter) {
 		return headers.entrySet().stream()
 				.map(entry -> entry.getKey() + ": " + entry.getValue())