title: NEST is adding TimeZone while indexing docs in Elasticsearch
body: <p>I have a DateTime field in my c# class as below</p>
  
  <pre><code> public DateTime PassedCreatedDate { get; set; }
  </code></pre>
  
  <p>While indexing it from NEST to elasticssearch, it is saving it along with local timezone. How to avoid this?</p>
  
  <pre><code> "PassedCreatedDate": "2015-08-14T15:50:04.0479046+05:30" //Actual value saved in ES
   "PassedCreatedDate": "2015-08-14T15:50:04.047" //Expected value
  </code></pre>
  
  <p>mapping of PassedCreatedDate in elasticsearch is</p>
  
  <pre><code>  "PassedCreatedDate": {
                    "type": "date",
                    "format": "dateOptionalTime"
                 },
  </code></pre>
  
  <p>I am aware to have a field as string and provide the format in ElasticProperty, but is there any setting to avoid this timezone addition while using datetime field only?</p>