title: ElasticSearch doubles my _id field when filtering on it
body: <p>I don't understand why ES doubles my _id field in an array when I'm filtering on it.</p>
  
  <pre>
  curl -X GET "http://localhost:9200/pgep-development_broadcasts/broadcast/_search?pretty=true" -d '{"query":{"match_all":{}},"fields":["_id", "title"]}'
  {
    "took" : 7,
    "timed_out" : false,
    "_shards" : {
      "total" : 5,
      "successful" : 5,
      "failed" : 0
    },
    "hits" : {
      "total" : 1,
      "max_score" : 1.0,
      "hits" : [ {
        "_index" : "pgep-development_broadcasts",
        "_type" : "broadcast",
        "_id" : "50ed959dcc93282abc000062",
        "_score" : 1.0,
        "fields" : {
          "_id" : [ "50ed959dcc93282abc000062", "50ed959dcc93282abc000062" ],
          "title" : "24 heures d'info"
        }
      } ]
    }
  }
  </pre>