title: elasticsearch returns null on stats aggregation
body: <p>I have small data of 1200 entries in Elasticsearch that is automatically input in mapped fields of document-types. The float goes in float and double goes in double.
  When taking 'aggs' of the data on 'stats' like:</p>
  
  <pre><code>GET /statsd-2015.09.28/timer_data/_search
  {
      "query" : {
          "filtered" : {
              "query" : { "match_all" : {}},
                  "filter" : {
                      "range" : { "ns" : { "lte" : "gunicorn" }}
                  }
              }
          },
          "aggs" : {
              "value_val" : { "stats" : { "field" : "u'count_90'" } }
          }
  }
  </code></pre>
  
  <p>I get null on return like this:</p>
  
  <pre><code>...  
  "aggregations": {
        "value_val": {
           "count": 0,
           "min": null,
           "max": null,
           "avg": null,
           "sum": null
        }
     }
  ...
  </code></pre>
  
  <p>Here is my mapping of fields:</p>
  
  <pre><code>{"statsd-2015.09.28":{"mappings":{"timer":{"properties":{"@timestamp":{"type":"string"},"act":{"type":"string"},"grp":{"type":"string"},"ns":{"type":"string"},"tgt":{"type":"string"},"val":{"type":"float"}}},"gauge":{"properties":{"@timestamp":{"type":"string"},"act":{"type":"string"},"grp":{"type":"string"},"ns":{"type":"string"},"tgt":{"type":"string"},"val":{"type":"float"}}},"counter":{"properties":{"@timestamp":{"type":"string"},"act":{"type":"string"},"grp":{"type":"string"},"ns":{"type":"string"},"tgt":{"type":"string"},"val":{"type":"float"}}},"timer_data":{"properties":{"@timestamp":{"type":"double"},"act":{"type":"string"},"count":{"type":"float"},"count_90":{"type":"float"},"count_ps":{"type":"float"},"grp":{"type":"string"},"lower":{"type":"float"},"mean":{"type":"float"},"mean_90":{"type":"float"},"median":{"type":"float"},"ns":{"type":"string"},"std":{"type":"float"},"sum":{"type":"float"},"sum_90":{"type":"float"},"sum_squares":{"type":"float"},"sum_squares_90":{"type":"float"},"tgt":{"type":"string"},"upper":{"type":"float"},"upper_90":{"type":"float"}}}}}}
  </code></pre>
  
  <p>What I want to ask is that why is my output not desired? And how can I get it?</p>