title: logstash mutate filter always stringifies hash and array
body: <p>I have a json log file, which I am taking as a input with this config</p>
  
  <pre><code>input {
     file { filename }
     codec { json_lines }
  }
  </code></pre>
  
  <p>Each line is a deeply nested JSON. </p>
  
  <p>In the filters,</p>
  
  <p>When I say </p>
  
  <pre><code>mutate { add_field =&gt; { "new_field_name" =&gt; "%{old_field_name}"}
  </code></pre>
  
  <ul>
  <li>if the old_field is a nested hash/array it is converted to a string and then added. Is there anyway I can preserve the type instead of stringifying it ?</li>
  </ul>