title: Response from interactive button POST is ignored in Mattermost
body: <p>I'm building a slash command. The flow I imagine is:</p>
  
  <ol>
  <li>User triggers slash command in client </li>
  <li>My server handles request and returns interactive dropdown</li>
  <li>User selects option from dropdown in client </li>
  <li>My server handles the POST request from the selection and
  returns an interactive button </li>
  <li>User presses the button</li>
  </ol>
  
  <p>Something seems to go wrong between steps 4 and five. The server handles the selection and returns the desired response:</p>
  
  <pre><code>{ 
    "update": {"message": "Updated!"}, 
    "ephemeral_text": "You updated the post!", 
    "attachments": [{
      "text": "Ready?", "actions": [{
        "name": "Go!", 
        "integration": {
          "url": "https://somewhere.com?foo=bar"
          "context": {"action": "event_submitted"}
        }
      }]
    }]
  }
  </code></pre>
  
  <p>...but nothing shows up in the client.</p>
  
  <p>Is something wrong with that JSON? I have tried other things like only returning a text message. </p>
  
  <p>I wonder, if I have misunderstood the docs and the client will never even evaluate a response from a POST triggered by an interactive message. In that case I would have to make a new request back from my server to the Mattermost API in order to get to the next step in my workflow. Is that correct?</p>