title: Slack API chat.update returns 'not_authed' error
body: <p>I have a <em>google script</em> running as a <em>webapp</em> to handle the backend of a <em>slack app</em>.</p>
  
  <p>The app has been <em>Authenticated</em> and I have the OAUTH <strong>token</strong> from this.</p>
  
  <p><em>I can currently post to a channel with button actions using the <strong>chat.postMessage</strong> using the for-mentioned <strong>token</strong>.</em></p>
  
  <p>Actions url points back at my webapp and hook in via doGet, from this response i construct a JSON object.</p>
  
  <pre><code>var response_payload = {
      "token" : access_token,
      "ts" : message_ts,
      "channel" : channel_id,
      "text" : "Approved! you are a winner!"
    })
  
  response_url = "https://slack.com/api/chat.update";
  
  sendToSlack_(response_url, response_payload)
  </code></pre>
  
  <p>posted via the following function:</p>
  
  <pre><code>function sendToSlack_(url,payload) {
     var options =  {
      "method" : "post",
      "contentType" : "application/json;charset=iso-8859-1",
      "payload" : JSON.stringify(payload)
    };
    return UrlFetchApp.fetch(url, options)
  }
  </code></pre>
  
  <p>however returned is the following:</p>
  
  <pre><code>{"ok":false,"error":"not_authed"}
  </code></pre>
  
  <p>I can't find any documentation about this error other than the following</p>
  
  <p><a href="https://stackoverflow.com/questions/31905260/sending-json-to-slack-in-a-http-post-request/33310661">Sending JSON to Slack in a HTTP POST request</a></p>
  
  <p>However this is in regard to a chat.postMessage request of which in my implementation is working correctly.</p>