title: null_value mapping in Elasticsearch
body: <p>I have created a mapping for a <code>tweetb</code> type in a <code>twitter</code> index:</p>
  
  <pre><code>curl -XPUT http://www.mydomain:9200/twitter/tweetb/_mapping -d '{
    "twitter": {
      "mappings": {
        "tweetb": {
          "properties": {
            "message": {
              "type": "string",
              "null_value": "NA"
            }
          }
        }
      }
    }
  }'
  </code></pre>
  
  <p>Then, I put one document:</p>
  
  <pre><code>curl -XPUT http://www.mydomain.com:9200/twitter/tweetb/1 -d '{"message": null}'
  </code></pre>
  
  <p>Then, I tried to get the inserted doc back:</p>
  
  <pre><code>curl -XGET http://www.mydomain:9200/twitter/tweetb/1
  </code></pre>
  
  <p>And that returned:</p>
  
  <pre><code>{
    "_index": "twitter",
    "_type": "tweetb",
    "_id": "1",
    "_version": 2,
    "found" : true,
    "_source" : { "message": null }
  }
  </code></pre>
  
  <p>I was expecting <code>"message" : "NA"</code> in the <code>_source</code> field. However, it looks like <code>"null_value"</code> isn't working.  Am I missing something?</p>