title: how to add a new field into a document using painless script
body: <p>is there a way of creating a field in document within a painless script
  if it does not exists?</p>
  
  <p>i'm using something like:</p>
  
  <pre><code>     if(!ctx._source.tags.contains(....)
  </code></pre>
  
  <p>but tags field may not be exists at document</p>
  
  <p>can it be done?</p>
  
  <p>thanks.</p>