make gzipResponseWriter implement http.Pusher (#1615)

Author: Warashi

middleware/compress.go

@@ -119,3 +119,10 @@ func (w *gzipResponseWriter) Flush() {
 func (w *gzipResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	return w.ResponseWriter.(http.Hijacker).Hijack()
 }
+
+func (w *gzipResponseWriter) Push(target string, opts *http.PushOptions) error {
+	if p, ok := w.ResponseWriter.(http.Pusher); ok {
+		return p.Push(target, opts)
+	}
+	return http.ErrNotSupported
+}