Always copy ServerResponse headers

Prior to this commit, ServerResponse headers would only be written if
there were no existing headers with the same name, thus making it
impossible to overwrite existing headers.

With the changes in this commit, headers are always written.

Closes gh-27741

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

@@ -362,9 +362,7 @@ 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() {