anchor.diff:          "title": "Hello world!",
           "image_url": "http://example.com/example.png",
           "subtitle": "This is an example"
  -      }],
  -      "buttons": [{
  -        "type": "web_url",
  -        "url": "http://example.com",
  -        "title": "Show website"
         }]
       }
     }
  
positive.code:               </td>
                <td>
                  <strong>array</strong> <span class="tag tag-success">Required</span>
                  <p>A collection of objects.</p>
                </td>
              </tr>
            </tbody>
  
negative.code:               </td>
                <td>
                  <strong>array</strong> <span class="tag tag-success">Required</span>
                  <p>A collection of <code>element</code> objects.</p>
                </td>
              </tr>
              <tr>
  