[HAWNG-641] Set text/plain content-type for "refresh" response. No ne…

…ed for Content-Security-Policy header (#3442) (#3444)

(cherry picked from commit 2bbd30d)

hawtio-system/src/main/java/io/hawt/web/auth/SessionExpiryFilter.java

@@ -70,7 +70,7 @@ public void doFilter(ServletRequest servletRequest, ServletResponse servletRespo
     }
 
     private void writeOk(HttpServletResponse response) throws IOException {
-        response.setContentType("text/html;charset=UTF-8");
+        response.setContentType("text/plain;charset=UTF-8");
         try (OutputStream out = response.getOutputStream()) {
             out.write("ok".getBytes());
             out.flush();