title: use max/min aggregation on a geo_point field type
body: <p>Is it possible to use a max or min aggregation on a geo_point field?</p>
  
  <p>I've tried to <code>max</code> directly onto my coordinate property which is of type <code>geo_point</code></p>
  
  <pre><code>{
      "size": 0,
      "aggs" : {
          "max_lat" : { "max" : { "field" : "coordinate" } }
      }
  }
  </code></pre>
  
  <p>this understandably returns a <code>ClassCastException</code> and so I tried to run the query directly on the <code>lat</code> or <code>lon</code> fields of <code>coordinate</code> which are of type double.</p>
  
  <pre><code>{
      "size": 0,
      "aggs" : {
          "max_lat" : { "max" : { "field" : "coordinate.lat" } },
          "max_lon" : { "max" : { "field" : "coordinate.lon" } }
      }
  }
  </code></pre>
  
  <p>Now I don't receive the ClassCastExcxeption and the query returns a 200 however the aggregations are null.</p>
  
  <p>Response:</p>
  
  <pre><code>{
      "aggregations": {
          "max_lat": {
              "value": null
          },
          "max_lon": {
              "value": null
          }
      }
  }
  </code></pre>
  
  <p>Using elasticsearch v1.7</p>