Fix JSON serialization of media_attachment (regression from #4090)

[unarist]

{
  "id":20,
  "type":"image",
  "url":"http://localhost:3000/system/media_attachments/files/000/000/020/original/501dc585d56426dd.png?1499464144",
  "preview_url":"http://localhost:3000/system/media_attachments/files/000/000/020/small/501dc585d56426dd.png?1499464144",
  "remote_url":"",
  "text_url":"http://localhost:3000/media/20",
  "meta":{ ... }
}

text_url should be https://.../media/{shortcode}, not https://.../media/{id}. Also it should be nil if it's remote resource (it throws Exception due to lack of shortcode).