Allow ResponseBodyAdvice to implement RequestBodyAdvice

Prior to this commit, if a @ControllerAdvice bean implemented both
RequestBodyAdvice and ResponseBodyAdvice, it was only supported as
RequestBodyAdvice, meaning it was never invoked as ResponseBodyAdvice.

This commit revises RequestResponseBodyAdviceChain to ensure that a
single bean implementing both types of body advice is in fact handled
as both types of advice.

See gh-22638

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestResponseBodyAdviceChain.java

 /*
- * Copyright 2002-2017 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.
@@ -37,6 +37,7 @@ import org.springframework.web.method.ControllerAdviceBean;
  * {@link org.springframework.web.method.ControllerAdviceBean ControllerAdviceBean}.
  *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  * @since 4.2
  */
 class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyAdvice<Object> {
@@ -48,7 +49,7 @@ class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyA
 
 	/**
 	 * Create an instance from a list of objects that are either of type
-	 * {@code ControllerAdviceBean} or {@code RequestBodyAdvice}.
+	 * {@code RequestBodyAdvice} or {@code ResponseBodyAdvice}.
 	 */
 	public RequestResponseBodyAdviceChain(List<Object> requestResponseBodyAdvice) {
 		initAdvice(requestResponseBodyAdvice);
@@ -64,7 +65,7 @@ class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyA
 			if (RequestBodyAdvice.class.isAssignableFrom(beanType)) {
 				this.requestBodyAdvice.add(advice);
 			}
-			else if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
+			if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
 				this.responseBodyAdvice.add(advice);
 			}
 		}