Add missing flush to submit headers

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/servlet/InstancesProxyController.java

@@ -67,7 +67,7 @@ public class InstancesProxyController extends AbstractInstancesProxyController {
     @RequestMapping(path = REQUEST_MAPPING_PATH, method = {RequestMethod.GET, RequestMethod.HEAD, RequestMethod.POST, RequestMethod.PUT, RequestMethod.PATCH, RequestMethod.DELETE, RequestMethod.OPTIONS})
     public Mono<Void> endpointProxy(@PathVariable("instanceId") String instanceId,
                                     HttpServletRequest servletRequest,
-                                    HttpServletResponse servletResponse) {
+                                    HttpServletResponse servletResponse) throws IOException {
         ServerHttpRequest request = new ServletServerHttpRequest(servletRequest);
         ServerHttpResponse response = new ServletServerHttpResponse(servletResponse);
 
@@ -88,6 +88,7 @@ public class InstancesProxyController extends AbstractInstancesProxyController {
 
         response.setStatusCode(clientResponse.statusCode());
         response.getHeaders().addAll(filterHeaders(clientResponse.headers().asHttpHeaders()));
+        response.flush();
 
         return clientResponse.body(BodyExtractors.toDataBuffers()).window(1).flatMap(body -> {
             try {

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/AbstractInstancesProxyControllerTest.java → spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/AbstractInstancesProxyControllerIntegrationTest.java

@@ -49,7 +49,7 @@ import static de.codecentric.boot.admin.server.utils.MediaType.ACTUATOR_V2_MEDIA
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.http.HttpHeaders.CONTENT_TYPE;
 
-public abstract class AbstractInstancesProxyControllerTest {
+public abstract class AbstractInstancesProxyControllerIntegrationTest {
     private WebTestClient client;
     private ConfigurableApplicationContext context;
     private ParameterizedTypeReference<Map<String, Object>> RESPONSE_TYPE = new ParameterizedTypeReference<Map<String, Object>>() {

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/reactive/InstancesProxyControllerIntegrationTest.java

@@ -17,13 +17,13 @@
 package de.codecentric.boot.admin.server.web.reactive;
 
 import de.codecentric.boot.admin.server.AdminReactiveApplicationTest;
-import de.codecentric.boot.admin.server.web.AbstractInstancesProxyControllerTest;
+import de.codecentric.boot.admin.server.web.AbstractInstancesProxyControllerIntegrationTest;
 
 import org.springframework.boot.WebApplicationType;
 import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.context.ConfigurableApplicationContext;
 
-public class InstancesProxyControllerIntegrationTest extends AbstractInstancesProxyControllerTest {
+public class InstancesProxyControllerIntegrationTest extends AbstractInstancesProxyControllerIntegrationTest {
     @Override
     protected ConfigurableApplicationContext setupContext() {
         return new SpringApplicationBuilder().sources(AdminReactiveApplicationTest.TestAdminApplication.class)

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/servlet/InstancesProxyControllerIntegrationTest.java

@@ -17,13 +17,13 @@
 package de.codecentric.boot.admin.server.web.servlet;
 
 import de.codecentric.boot.admin.server.AdminServletApplicationTest;
-import de.codecentric.boot.admin.server.web.AbstractInstancesProxyControllerTest;
+import de.codecentric.boot.admin.server.web.AbstractInstancesProxyControllerIntegrationTest;
 
 import org.springframework.boot.WebApplicationType;
 import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.context.ConfigurableApplicationContext;
 
-public class InstancesProxyControllerIntegrationTest extends AbstractInstancesProxyControllerTest {
+public class InstancesProxyControllerIntegrationTest extends AbstractInstancesProxyControllerIntegrationTest {
     @Override
     protected ConfigurableApplicationContext setupContext() {
         return new SpringApplicationBuilder().sources(AdminServletApplicationTest.TestAdminApplication.class)