[Feature] add path prefix for http server (#2439)

* update http.NewServer
Add feature:option of mux router prefix path.

* remove nil check in
transport->http->PathPrefix option.

Co-authored-by: luozhihong <luozhihong@risenlighten.com>

transport/http/server.go

@@ -128,6 +128,13 @@ func Listener(lis net.Listener) ServerOption {
 	}
 }
 
+// PathPrefix with mux's PathPrefix, router will replaced by a subrouter that start with prefix.
+func PathPrefix(prefix string) ServerOption {
+	return func(s *Server) {
+		s.router = s.router.PathPrefix(prefix).Subrouter()
+	}
+}
+
 // Server is an HTTP server wrapper.
 type Server struct {
 	*http.Server
@@ -162,11 +169,12 @@ func NewServer(opts ...ServerOption) *Server {
 		enc:         DefaultResponseEncoder,
 		ene:         DefaultErrorEncoder,
 		strictSlash: true,
+		router:      mux.NewRouter(),
 	}
 	for _, o := range opts {
 		o(srv)
 	}
-	srv.router = mux.NewRouter().StrictSlash(srv.strictSlash)
+	srv.router.StrictSlash(srv.strictSlash)
 	srv.router.NotFoundHandler = http.DefaultServeMux
 	srv.router.MethodNotAllowedHandler = http.DefaultServeMux
 	srv.router.Use(srv.filter())