title: add new field to existing documents by update in elasticsearch
body: <p>I have an <code>index</code> with properties defined but in documents there is one property missing while importing from another elasticsearch.</p>
  
  <p>Say, in index the mapping properties are:</p>
  
  <pre><code>"properties": { 
      "name": {"type" : ..},
      "address": {"type" : ..},
      "email": {"type" : ..}
  }
  </code></pre>
  
  <p>But in documents, one of the property say <code>email</code> is missing as:</p>
  
  <pre><code>{
      "name": "Wheeler Walker",
      "address": "Texas",
  },
  {
      "name": "Kenny Rogers",
      "address": "Virginia",
  }
  </code></pre>
  
  <p>I have the property <code>email</code> defined in <code>mapping</code> already.</p>
  
  <p>Now I just need to bulk update these documents so that <code>email</code> property has <code>""</code> empty value.</p>
  
  <p>How can I do that?</p>