header: implement http.Hijacker for responseWriterWrapper

fix issue #1173

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/header/header.go

@@ -4,6 +4,9 @@
 package header
 
 import (
+	"bufio"
+	"errors"
+	"net"
 	"net/http"
 	"strings"
 
@@ -113,3 +116,12 @@ func (rww *responseWriterWrapper) setHeader(key, value string) {
 		h.Set(key, value)
 	})
 }
+
+// Hijack implements http.Hijacker. It simply wraps the underlying
+// ResponseWriter's Hijack method if there is one, or returns an error.
+func (rww *responseWriterWrapper) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if hj, ok := rww.w.(http.Hijacker); ok {
+		return hj.Hijack()
+	}
+	return nil, nil, errors.New("not a Hijacker")
+}

caddyhttp/proxy/reverseproxy.go

@@ -189,7 +189,7 @@ func (rp *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request,
 		res.Body.Close()
 		hj, ok := rw.(http.Hijacker)
 		if !ok {
-			return nil
+			panic("not a hijacker")
 		}
 
 		conn, _, err := hj.Hijack()