public function addDefaultVisitors(): Http20Builder
    {
        if ($this->addedDefaultVisitors) {
            return $this;
        }

        $this->addedDefaultVisitors = true;

        $this->addVisitor(new AddExpirationHeaderVisitor());
        $this->addVisitor(new AddPriorityHeaderVisitor());
        $this->addVisitor(new AddApnIdHeaderVisitor());
        $this->addVisitor(new AddCollapseIdHeaderVisitor());

        return $this;
    }