internal handlers: support for response modifiers

Co-authored-by: Julien Salleyron <julien@containo.us>
Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Jean-Baptiste Doumenjou <jb.doumenjou@gmail.com>

integration/fixtures/headers/secure.toml

@@ -2,6 +2,9 @@
   checkNewVersion = false
   sendAnonymousUsage = false
 
+[api]
+  insecure = true
+
 [log]
   level = "DEBUG"
 
@@ -24,6 +27,11 @@
     rule = "Host(`test2.localhost`)"
     service = "service1"
 
+  [http.routers.router3]
+    rule = "Host(`internal.localhost`)"
+    middlewares = ["secure"]
+    service = "api@internal"
+
 [http.middlewares]
   [http.middlewares.secure.headers]
     featurePolicy = "vibrate 'none';"