Allow ServerHttpRequest content-type mutation

Prior to this commit, `ServerHttpRequest.mutate()` would not reflect
changes made on the "Accept" and "Content-Type" HTTP headers.
This was due to the fact that the instantiation of a new request based
on the mutated values would not use the writable HTTP headers used
during the mutation, but rather a read-only view of the headers backed
by `ReadOnlyHttpHeaders`.

`ReadOnlyHttpHeaders` caches those values for performance reasons, so
getting those from the new request would not reflect the changes made
during the mutation phase.

This commit ensures that the new request uses the mutated headers.

Fixes spring-projectsgh-26615

spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java

@@ -38,6 +38,7 @@
  *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
+ * @author Brian Clozel
  * @since 5.0
  */
 class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
@@ -131,7 +132,7 @@ public ServerHttpRequest.Builder remoteAddress(InetSocketAddress remoteAddress)
 	@Override
 	public ServerHttpRequest build() {
 		return new MutatedServerHttpRequest(getUriToUse(), this.contextPath,
-				this.httpMethodValue, this.sslInfo, this.remoteAddress, this.body, this.originalRequest);
+				this.httpMethodValue, this.sslInfo, this.remoteAddress, this.headers, this.body, this.originalRequest);
 	}
 
 	private URI getUriToUse() {
@@ -190,9 +191,9 @@ private static class MutatedServerHttpRequest extends AbstractServerHttpRequest
 
 		public MutatedServerHttpRequest(URI uri, @Nullable String contextPath,
 				String methodValue, @Nullable SslInfo sslInfo, @Nullable InetSocketAddress remoteAddress,
-				Flux<DataBuffer> body, ServerHttpRequest originalRequest) {
+				HttpHeaders headers, Flux<DataBuffer> body, ServerHttpRequest originalRequest) {
 
-			super(uri, contextPath, originalRequest.getHeaders());
+			super(uri, contextPath, headers);
 			this.methodValue = methodValue;
 			this.remoteAddress = (remoteAddress != null ? remoteAddress : originalRequest.getRemoteAddress());
 			this.sslInfo = (sslInfo != null ? sslInfo : originalRequest.getSslInfo());

spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java

@@ -30,6 +30,7 @@
 
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.testfixture.servlet.DelegatingServletInputStream;
 import org.springframework.web.testfixture.servlet.MockAsyncContext;
@@ -46,6 +47,7 @@
  *
  * @author Rossen Stoyanchev
  * @author Sam Brannen
+ * @author Brian Clozel
  */
 public class ServerHttpRequestTests {
 
@@ -166,6 +168,18 @@ public void mutateHeaderBySettingHeaderValues() throws Exception {
 		assertThat(request.getHeaders().get(headerName)).containsExactly(headerValue3);
 	}
 
+	@Test // gh-26615
+	void mutateContentTypeHeaderValue() throws Exception {
+		ServerHttpRequest request = createRequest("/path").mutate()
+				.headers(headers -> headers.setContentType(MediaType.APPLICATION_JSON)).build();
+
+		assertThat(request.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_JSON);
+
+		ServerHttpRequest mutated = request.mutate()
+				.headers(headers -> headers.setContentType(MediaType.APPLICATION_CBOR)).build();
+		assertThat(mutated.getHeaders().getContentType()).isEqualTo(MediaType.APPLICATION_CBOR);
+	}
+
 	@Test
 	void mutateWithExistingContextPath() throws Exception {
 		ServerHttpRequest request = createRequest("/context/path", "/context");