Use ServletResponse#getContentType in ServletServerHttpResponse

This commit updates ServletServerHttpResponse.ServletResponseHttpHeaders
in order to use ServletResponse#getContentType instead of
ServletResponse#getHeader.

It allows to have a consistent behavior between Tomcat (which set only
the former) and Undertow/Jetty (which set both).

See spring-projectsgh-32339

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -27,6 +27,7 @@
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatusCode;
+import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
@@ -36,6 +37,7 @@
  *
  * @author Arjen Poutsma
  * @author Rossen Stoyanchev
+ * @author Sebastien Deleuze
  * @since 3.0
  */
 public class ServletServerHttpResponse implements ServerHttpResponse {
@@ -159,8 +161,8 @@ public boolean containsKey(Object key) {
 		public String getFirst(String headerName) {
 			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
 				// Content-Type is written as an override so check super first
-				String value = super.getFirst(headerName);
-				return (value != null ? value : servletResponse.getHeader(headerName));
+				String contentType = super.getFirst(headerName);
+				return (contentType != null ? contentType : servletResponse.getContentType());
 			}
 			else {
 				String value = servletResponse.getHeader(headerName);
@@ -175,7 +177,8 @@ public List<String> get(Object key) {
 			String headerName = (String) key;
 			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
 				// Content-Type is written as an override so don't merge
-				return Collections.singletonList(getFirst(headerName));
+				String contentType = getFirst(headerName);
+				return Collections.singletonList(contentType != null ? contentType : servletResponse.getContentType());
 			}
 
 			Collection<String> values1 = servletResponse.getHeaders(headerName);