title: Data missing in extra_fields in /collectors/{id}/recipients
body: <p>I am trying to get the response with this query from SurveyMoneky:</p>
  
  <pre><code>/collectors/{id}/recipients?include=survey_response_status,mail_status,custom_fields,remove_link,extra_fields,survey_link 
  </code></pre>
  
  <p>Response I am getting is: </p>
  
  <pre><code>"last_name": "Siraj",
  "first_name": "Abdullah",
  "survey_response_status": "completely_responded",
  "mail_status": "sent",
  "email": "***",
  "href": "****",
  "custom_fields": {
      "1": null,
      "6": "2017"
  },
  "remove_link": "****",
  "id": "3310118350",
  "survey_link": "****"
  </code></pre>
  
  <p>My problem is: </p>
  
  <ol>
  <li>I am supposed to get the extra_fields which I am not receiving </li>
  <li>custom_field 1 is definitely not null, its wrong data. </li>
  </ol>
  
  <p>The actual values of those fields are: </p>
  
  <pre><code>"extra_fields": {
      "job_item_id": "123456",
      "year": "2017"
  },
  
  "custom_fields": {
      "1": "123456",
      "6": "2017"
  }
  </code></pre>
  
  <p>I checked with this call: <code>/collectors/{id}/recipients/{id}</code> and this works fine. </p>
  
  <p>Am I missing something?</p>