ResponseEntityResultHandler overwrites headers

Prior to this commit, controller handlers (regular and exception
handlers as well) would not overwrite existing HTTP response headers on
the exchange. This would lead to situations where Content-Type values
set during the initial handling phase would not be overwritten when
handling an error later on.

This commit aligns the implementation of that result handler on the
Spring MVC one in that regard.

Issue: SPR-17082
(Cherry-picked from 195f3f07e7)

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandler.java

@@ -155,7 +155,6 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 			HttpHeaders responseHeaders = exchange.getResponse().getHeaders();
 			if (!entityHeaders.isEmpty()) {
 				entityHeaders.entrySet().stream()
-						.filter(entry -> !responseHeaders.containsKey(entry.getKey()))
 						.forEach(entry -> responseHeaders.put(entry.getKey(), entry.getValue()));
 			}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -44,6 +44,7 @@ import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.HttpMessageWriter;
@@ -334,6 +335,22 @@ public class ResponseEntityResultHandlerTests {
 		assertResponseBodyIsEmpty(exchange);
 	}
 
+	@Test // SPR-17082
+	public void handleResponseEntityWithExistingResponseHeaders() throws Exception {
+		ResponseEntity<Void> value = ResponseEntity.ok().contentType(MediaType.APPLICATION_JSON).build();
+		MethodParameter returnType = on(TestController.class).resolveReturnType(entity(Void.class));
+		HandlerResult result = handlerResult(value, returnType);
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));
+		exchange.getResponse().getHeaders().setContentType(MediaType.TEXT_PLAIN);
+		this.resultHandler.handleResult(exchange, result).block(Duration.ofSeconds(5));
+
+		assertEquals(HttpStatus.OK, exchange.getResponse().getStatusCode());
+		assertEquals(1, exchange.getResponse().getHeaders().size());
+		assertEquals(MediaType.APPLICATION_JSON, exchange.getResponse().getHeaders().getContentType());
+		assertResponseBodyIsEmpty(exchange);
+	}
+
+
 
 	private void testHandle(Object returnValue, MethodParameter returnType) {
 		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));