Fix missing ETag/LastModified headers in responses

Prior to this commit, the `HttpEntityMethodProcessor` would avoid
writing ETag/Last-Modified response headers before calling
`ServletWebRequest` to process conditional requests. This was done to
avoid duplicate response header values due to headers being already
written to the underlying servlet response.

This is still necessary for GET/HEAD requests, since this is properly
handled by `ServletWebRequest` for those cases. But
`HttpEntityMethodProcessor` should not make that decision for
PUT/PATCH/POST responses since developers are adding response headers on
purpose and should be in control of the situation — whereas
`ServletWebRequest` does not write those headers in those cases.

Issue: SPR-14767

Author: bclozel

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

@@ -28,6 +28,7 @@
 import org.springframework.core.ResolvableType;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.HttpMessageConverter;
@@ -183,7 +184,7 @@ public void handleReturnValue(Object returnValue, MethodParameter returnType,
 		if (responseEntity instanceof ResponseEntity) {
 			int returnStatus = ((ResponseEntity<?>) responseEntity).getStatusCodeValue();
 			outputMessage.getServletResponse().setStatus(returnStatus);
-			if(returnStatus == 200) {
+			if (returnStatus == 200) {
 				if (isResourceNotModified(inputMessage, outputMessage)) {
 					// Ensure headers are flushed, no body should be written.
 					outputMessage.flush();
@@ -227,8 +228,10 @@ private boolean isResourceNotModified(ServletServerHttpRequest inputMessage, Ser
 		HttpHeaders responseHeaders = outputMessage.getHeaders();
 		String etag = responseHeaders.getETag();
 		long lastModifiedTimestamp = responseHeaders.getLastModified();
-		responseHeaders.remove(HttpHeaders.ETAG);
-		responseHeaders.remove(HttpHeaders.LAST_MODIFIED);
+		if (inputMessage.getMethod() == HttpMethod.GET || inputMessage.getMethod() == HttpMethod.HEAD) {
+			responseHeaders.remove(HttpHeaders.ETAG);
+			responseHeaders.remove(HttpHeaders.LAST_MODIFIED);
+		}
 
 		return servletWebRequest.checkNotModified(etag, lastModifiedTimestamp);
 	}