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 sets only
the former) and Undertow/Jetty (which set both).

Closes gh-32339

Author: sdeleuze

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

@@ -162,7 +162,7 @@ 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));
+				return (value != null ? value : servletResponse.getContentType());
 			}
 			else {
 				String value = servletResponse.getHeader(headerName);