caddyhttp: Make use of `http.ResponseController` (#5654)

* caddyhttp: Make use of http.ResponseController

Also syncs the reverseproxy implementation with stdlib's which now uses ResponseController as well https://github.com/golang/go/commit/2449bbb5e614954ce9e99c8a481ea2ee73d72d61

* Enable full-duplex for HTTP/1.1

* Appease linter

* Add warning for builds with Go 1.20, so it's less surprising to users

* Improved godoc for EnableFullDuplex, copied text from stdlib

* Only wrap in encode if not already wrapped

1 files changed, 25 insertions, 0 deletions

diff --git a/modules/caddyhttp/duplex_go121.go b/modules/caddyhttp/duplex_go121.go
new file mode 100644
index 0000000..4f3851c
--- /dev/null
+++ b/modules/caddyhttp/duplex_go121.go
@@ -0,0 +1,25 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build go1.21
+
+package caddyhttp
+
+import (
+	"net/http"
+)
+
+func enableFullDuplex(w http.ResponseWriter) {
+	http.NewResponseController(w).EnableFullDuplex()
+}