title: modify_shared_link_settings - how to remove expiration?
body: <p>I'm using v2 of the core API with HTTP directly, the method <a href="https://www.dropbox.com/developers/documentation/http/documentation#sharing-modify_shared_link_settings" rel="nofollow" title="modify_shared_link_settings">https://www.dropbox.com/developers/documentation/http/documentation#sharing-modify_shared_link_settings</a>. I have no problems changing the visiblity, password and expiration date of a link, but I can't see how to <em>remove</em> the expiration entirely.</p>
  
  <p>I've tried </p>
  
  <ul>
  <li>not sending 'expires' at all in my request while changing other properties - no effect</li>
  <li>setting 'expires' to the the JSON literal null - no effect</li>
  <li>setting 'expires' to a time in the past - gives a 'invalid_settings' error</li>
  <li>setting 'expires' to an empty string - gives a "time data " " does not match format '%Y-%m-%dT%H:%M:%SZ'" error.</li>
  </ul>
  
  <p>I'm kind of at a loss here, is there something obvious I'm missing?</p>