Merge branch '5.3.x'

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultServerResponseBuilder.java

@@ -361,9 +361,7 @@ class DefaultServerResponseBuilder implements ServerResponse.BodyBuilder {
 
 		private static <K,V> void copy(MultiValueMap<K,V> src, MultiValueMap<K,V> dst) {
 			if (!src.isEmpty()) {
-				src.entrySet().stream()
-						.filter(entry -> !dst.containsKey(entry.getKey()))
-						.forEach(entry -> dst.put(entry.getKey(), entry.getValue()));
+				dst.putAll(src);
 			}
 		}
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/DefaultServerResponseBuilderTests.java

@@ -320,6 +320,23 @@ public class DefaultServerResponseBuilderTests {
 		assertThat(serverResponse.block().cookies().isEmpty()).isFalse();
 	}
 
+	@Test
+	public void overwriteHeaders() {
+		ServerResponse serverResponse =
+				ServerResponse.ok().headers(headers -> headers.set("Foo", "Bar")).build().block();
+		assertThat(serverResponse).isNotNull();
+
+		MockServerWebExchange mockExchange = MockServerWebExchange
+				.builder(MockServerHttpRequest.get("https://example.org"))
+				.build();
+		MockServerHttpResponse response = mockExchange.getResponse();
+		response.getHeaders().set("Foo", "Baz");
+
+		serverResponse.writeTo(mockExchange, EMPTY_CONTEXT).block();
+
+		assertThat(response.getHeaders().getFirst("Foo")).isEqualTo("Bar");
+	}
+
 
 	@Test
 	public void build() {