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>

pkg/responsemodifiers/response_modifier_test.go

@@ -184,6 +184,9 @@ func TestBuilderBuild(t *testing.T) {
 			builder := NewBuilder(rtConf.Middlewares)
 
 			rm := builder.Build(context.Background(), test.middlewares)
+			if rm == nil {
+				return
+			}
 
 			resp := test.buildResponse(test.conf)