Fix handling of headers in reactive HTTP requests to ensure case-insensivity

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

@@ -30,7 +30,6 @@
 import org.springframework.http.HttpMethod;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
-import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 import org.springframework.util.StringUtils;
 
@@ -72,7 +71,9 @@ public DefaultServerHttpRequestBuilder(ServerHttpRequest original) {
 
 		this.uri = original.getURI();
 		// original headers can be immutable, so create a copy
-		this.headers = new HttpHeaders(new LinkedMultiValueMap<>(original.getHeaders()));
+		this.headers = new HttpHeaders();
+		this.headers.putAll(original.getHeaders());
+
 		this.httpMethod = original.getMethod();
 		this.contextPath = original.getPath().contextPath().value();
 		this.remoteAddress = original.getRemoteAddress();

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

@@ -212,6 +212,13 @@ public void mutateDoesNotPreventAccessToNativeRequest() throws Exception {
 		assertThat(nativeRequest).isInstanceOf(HttpServletRequest.class);
 	}
 
+	@Test // gh-33666
+	public void mutateKeepsHeadersCaseInsensitive() throws Exception {
+		ServerHttpRequest request = createRequest("/path");
+		request = request.mutate().header("key", "value").build();
+		assertThat(request.getHeaders().get("KEY")).isEqualTo(Collections.singletonList("value"));
+	}
+
 	private ServerHttpRequest createRequest(String uriString) throws Exception {
 		return createRequest(uriString, "");
 	}