title: how to serialize a json string to a form post data
body: <p>I need to POST a JSON string to a page.
  The page is external and out of my control, and it expects the post data to be in the web-form post format (<code>key1=value1&amp;key2=value2</code>)</p>
  
  <p>How can I convert the JSON string to this format?</p>