Handle message of @ResponseStatus-annotated exception with WebFlux

See gh-19901

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/reactive/error/DefaultErrorAttributes.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -24,6 +24,7 @@ import java.util.Map;
 
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.http.HttpStatus;
+import org.springframework.util.StringUtils;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.ObjectError;
 import org.springframework.web.bind.annotation.ResponseStatus;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/reactive/error/DefaultErrorAttributesTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -89,6 +89,18 @@ public class DefaultErrorAttributesTests {
 		Map<String, Object> attributes = this.errorAttributes.getErrorAttributes(buildServerRequest(request, error),
 				false);
 		assertThat(attributes.get("error")).isEqualTo(HttpStatus.I_AM_A_TEAPOT.getReasonPhrase());
+		assertThat(attributes.get("message")).isEqualTo("");
+		assertThat(attributes.get("status")).isEqualTo(HttpStatus.I_AM_A_TEAPOT.value());
+	}
+
+	@Test
+	void annotatedResponseStatusCodeWithExceptionMessage() {
+		Exception error = new CustomException("Test Message");
+		MockServerHttpRequest request = MockServerHttpRequest.get("/test").build();
+		Map<String, Object> attributes = this.errorAttributes.getErrorAttributes(buildServerRequest(request, error),
+				false);
+		assertThat(attributes.get("error")).isEqualTo(HttpStatus.I_AM_A_TEAPOT.getReasonPhrase());
+		assertThat(attributes.get("message")).isEqualTo("Test Message");
 		assertThat(attributes.get("status")).isEqualTo(HttpStatus.I_AM_A_TEAPOT.value());
 	}
 
@@ -218,6 +230,13 @@ public class DefaultErrorAttributesTests {
 	@ResponseStatus(HttpStatus.I_AM_A_TEAPOT)
 	private static class CustomException extends RuntimeException {
 
+		CustomException() {
+		}
+
+		CustomException(String message) {
+			super(message);
+		}
+
 	}
 
 	@ResponseStatus(value = HttpStatus.I_AM_A_TEAPOT, reason = "Nope!")