code: func (mx *Mux) buildRouteHandler() {
  	mx.handler = chain(mx.middlewares, http.HandlerFunc(mx.routeHTTP))
  }
docs: // buildRouteHandler builds the single mux handler that is a chain of the middleware
  // stack, as defined by calls to Use(), and the tree router (Mux) itself. After this
  // point, no other middlewares can be registered on this Mux's stack. But you can still
  // compose additional middlewares via Group()'s or using a chained middleware handler.