Fix stats responseRecorder Hijacker

Signed-off-by: Emile Vauge <emile@vauge.com>

middlewares/stateful.go

@@ -0,0 +1,12 @@
+package middlewares
+
+import "net/http"
+
+// Stateful interface groups all http interfaces that must be
+// implemented by a stateful middleware (ie: recorders)
+type Stateful interface {
+	http.ResponseWriter
+	http.Hijacker
+	http.Flusher
+	http.CloseNotifier
+}