Wrapper adds flushBuffer

Signed-off-by: Jorge Bescos Gascon <jorge.bescos.gascon@oracle.com>

ext/mvc-jsp/src/main/java/org/glassfish/jersey/server/mvc/jsp/JspTemplateProcessor.java

diff --git a/ext/mvc-jsp/src/main/java/org/glassfish/jersey/server/mvc/jsp/JspTemplateProcessor.java b/ext/mvc-jsp/src/main/java/org/glassfish/jersey/server/mvc/jsp/JspTemplateProcessor.java
index 755762f..84187c9 100644
--- a/ext/mvc-jsp/src/main/java/org/glassfish/jersey/server/mvc/jsp/JspTemplateProcessor.java
+++ b/ext/mvc-jsp/src/main/java/org/glassfish/jersey/server/mvc/jsp/JspTemplateProcessor.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013, 2018 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, 2020 Oracle and/or its affiliates. All rights reserved.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0, which is available at
@@ -124,6 +124,15 @@
                 public PrintWriter getWriter() throws IOException {
                     return responseWriter;
                 }
+
+                @Override
+                public void flushBuffer() throws IOException {
+                    /*
+                     *  Need to avoid different implementations to do something else, like invalidating new
+                     *  headers after flushBuffer, because later when ContainerResponse.closes it flushBuffer again.
+                     */
+                    getOutputStream().flush();
+                }
             });
         } catch (final Exception e) {
             throw new ContainerException(e);