title: how to create empty array?
body: <p>It's possible to get following output from boost::property_tree::json_parser::write_json</p>
  
  <pre><code>{
    "id": "id",
    "method": "test_method",
    "params": []
  }
  </code></pre>
  
  <p>I know how to create array with some items, but when I try to create empty array, I've get "params": ""</p>