chore(response): prevention of Hijack() runtime panics (#4295)

* Prevention of Hijack() runtime panics

* added test of Hijack()

* fix review

* fix lint error

* added check assertion of Wrrten() condition before calling Hijack()

response_writer.go

@@ -6,6 +6,7 @@ package gin
 
 import (
 	"bufio"
+	"errors"
 	"io"
 	"net"
 	"net/http"
@@ -16,6 +17,8 @@ const (
 	defaultStatus = http.StatusOK
 )
 
+var errHijackAlreadyWritten = errors.New("gin: response already written")
+
 // ResponseWriter ...
 type ResponseWriter interface {
 	http.ResponseWriter
@@ -106,6 +109,9 @@ func (w *responseWriter) Written() bool {
 
 // Hijack implements the http.Hijacker interface.
 func (w *responseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if w.Written() {
+		return nil, nil, errHijackAlreadyWritten
+	}
 	if w.size < 0 {
 		w.size = 0
 	}