Let hijacker and flusher interfaces to be reflected. (#3709)

Ideally here if the interface is not found it would
fail the server, as it should be because without these
we can't even have a working server in the first place.

Just like how it fails in master invariably inside Go
net/http code path.

Fixes #3708

cmd/generic-handlers.go

@@ -17,6 +17,8 @@
 package cmd
 
 import (
+	"bufio"
+	"net"
 	"net/http"
 	"path"
 	"strings"
@@ -361,7 +363,6 @@ func (h resourceHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // to record some useful http response data.
 type httpResponseRecorder struct {
 	http.ResponseWriter
-	http.Flusher
 	respStatusCode int
 }
 
@@ -372,10 +373,7 @@ func (rww *httpResponseRecorder) Write(b []byte) (int, error) {
 
 // Wraps ResponseWriter's Flush()
 func (rww *httpResponseRecorder) Flush() {
-	f, ok := rww.ResponseWriter.(http.Flusher)
+	rww.ResponseWriter.(http.Flusher).Flush()
-	if ok {
-		f.Flush()
-	}
 }
 
 // Wraps ResponseWriter's WriteHeader() and record
@@ -385,6 +383,10 @@ func (rww *httpResponseRecorder) WriteHeader(httpCode int) {
 	rww.ResponseWriter.WriteHeader(httpCode)
 }
 
+func (rww *httpResponseRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return rww.ResponseWriter.(http.Hijacker).Hijack()
+}
+
 // httpStatsHandler definition: gather HTTP statistics
 type httpStatsHandler struct {
 	handler http.Handler