title: Elasticsearch doesn't obey _mapping after reindex
body: <p>I have a field called <em>last_post_date</em> stored in elasticsearch.</p>
  
  <p>It's shown as:</p>
  
  <pre><code>...
  "last_post_date": "1485281760000",
  ...
  </code></pre>
  
  <p>And has the mapping as:</p>
  
  <pre><code>"last_post_date": {
      "type": "text",
      "fields": {
          "keyword": {
              "type": "keyword",
              "ignore_above": 256
          }
      }
  },
  </code></pre>
  
  <p>Now I create a new index with the mapping as:</p>
  
  <pre><code>...
  "last_post_date": {
      "type": "date",
      "format": "epoch_millis",
      "index": true,
      "doc_values": true
  },
  ...
  </code></pre>
  
  <p>And do a full reindex via:
  <a href="http://127.0.0.1:9200/_reindex" rel="nofollow noreferrer">http://127.0.0.1:9200/_reindex</a></p>
  
  <p>The problem is that even after reindexing the field's value is stored as string instead of date while the mapping shows:</p>
  
  <pre><code>...
  "last_post_date": {
      "type": "date",
      "store": true,
      "format": "epoch_millis"
  },
  ...
  </code></pre>
  
  <p>Any idea why?</p>