title: Delete field in doc using spring esTemplate
body: <p>I'm trying to use ElasticsearchTemplate to update my es data,code is as follows:</p>
  
  <pre><code>UpdateRequest updateRequest = new UpdateRequest();
              updateRequest.index(RiskViewDevStatistics.ESIndex);
              updateRequest.type(RiskViewDevStatistics.ESType);
              try {
                  updateRequest.doc(XContentFactory.jsonBuilder().startObject()
                          .field("owner", ownerId)
                          .endObject());
                  UpdateQuery updateQuery = new UpdateQueryBuilder().withId("docId")
                          .withClass(RiskViewDevStatistics.class).withUpdateRequest(updateRequest).build();
                  esTemplate.update(updateQuery);
  </code></pre>
  
  <p>The code is similar to : </p>
  
  <pre><code>update owner from xx where id = xx
  </code></pre>
  
  <p>When the argument ownerId I passed in is null,data in es is :</p>
  
  <pre><code>"hits": {
          "total": 1,
          "max_score": 1.0,
          "hits": [
              {
                  "_index": "st-riskview",
                  "_type": "riskviewdevstatistics",
                  "_id": "AW47Wy7cEY_O-MqaGSGL",
                  "_score": 1.0,
                  "_source": {
                      "owner": null  // null
                  }
              }
          ]
      }
  </code></pre>
  
  <p>My question is : 
  <strong>How to delete this field instead of setting it to null ?</strong> </p>
  
  <p>Attention:I don't want to replace total doc.</p>