Avoid duplicate Etag/Last-Modified header values

This commit improves SPR-13090 and avoids adding duplicate ETag and
Last-Modified headers in HTTP responses.
Previously, those were added twice to the response since:

* we're adding all ResponseEntity headers to the response
* the `checkNotModified` methods automatically add those headers

Issue: SPR-13090

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

@@ -168,12 +168,15 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 		}
 
 		HttpHeaders entityHeaders = responseEntity.getHeaders();
-		if (!entityHeaders.isEmpty()) {
-			outputMessage.getHeaders().putAll(entityHeaders);
-		}
 
 		Object body = responseEntity.getBody();
 		if (responseEntity instanceof ResponseEntity) {
+			for (String headerName : entityHeaders.keySet()) {
+				if(!HttpHeaders.LAST_MODIFIED.equals(headerName)
+					&& !HttpHeaders.ETAG.equals(headerName)) {
+					outputMessage.getHeaders().put(headerName, entityHeaders.get(headerName));
+				}
+			}
 			if (isResourceNotModified(webRequest, (ResponseEntity<?>) responseEntity)) {
 				// Ensure headers are flushed, no body should be written.
 				outputMessage.flush();
@@ -181,6 +184,11 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 				return;
 			}
 		}
+		else {
+			if (!entityHeaders.isEmpty()) {
+				outputMessage.getHeaders().putAll(entityHeaders);
+			}
+		}
 
 		// Try even with null body. ResponseBodyAdvice could get involved.
 		writeWithMessageConverters(body, returnType, inputMessage, outputMessage);

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessorMockTests.java

@@ -349,6 +349,7 @@ public class HttpEntityMethodProcessorMockTests {
 
 		assertTrue(mavContainer.isRequestHandled());
 		assertEquals(HttpStatus.NOT_MODIFIED.value(), servletResponse.getStatus());
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.LAST_MODIFIED).size());
 		assertEquals(dateFormat.format(oneMinuteAgo), servletResponse.getHeader(HttpHeaders.LAST_MODIFIED));
 		assertEquals(0, servletResponse.getContentAsByteArray().length);
 	}
@@ -369,6 +370,7 @@ public class HttpEntityMethodProcessorMockTests {
 
 		assertTrue(mavContainer.isRequestHandled());
 		assertEquals(HttpStatus.NOT_MODIFIED.value(), servletResponse.getStatus());
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.ETAG).size());
 		assertEquals(etagValue, servletResponse.getHeader(HttpHeaders.ETAG));
 		assertEquals(0, servletResponse.getContentAsByteArray().length);
 	}
@@ -393,7 +395,9 @@ public class HttpEntityMethodProcessorMockTests {
 
 		assertTrue(mavContainer.isRequestHandled());
 		assertEquals(HttpStatus.NOT_MODIFIED.value(), servletResponse.getStatus());
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.LAST_MODIFIED).size());
 		assertEquals(dateFormat.format(oneMinuteAgo), servletResponse.getHeader(HttpHeaders.LAST_MODIFIED));
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.ETAG).size());
 		assertEquals(etagValue, servletResponse.getHeader(HttpHeaders.ETAG));
 		assertEquals(0, servletResponse.getContentAsByteArray().length);
 	}
@@ -419,7 +423,9 @@ public class HttpEntityMethodProcessorMockTests {
 
 		assertTrue(mavContainer.isRequestHandled());
 		assertEquals(HttpStatus.OK.value(), servletResponse.getStatus());
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.LAST_MODIFIED).size());
 		assertEquals(dateFormat.format(oneMinuteAgo), servletResponse.getHeader(HttpHeaders.LAST_MODIFIED));
+		assertEquals(1, servletResponse.getHeaderValues(HttpHeaders.ETAG).size());
 		assertEquals(changedEtagValue, servletResponse.getHeader(HttpHeaders.ETAG));
 		assertEquals(0, servletResponse.getContentAsByteArray().length);
 	}