title: Missing Id field in Solr index
body: <p>I just find out that my Solr index doesn't contain a <em>_id</em> field. 
  and it is not possible to get item id. UniqueId is present but this is not really useful. </p>
  
  <pre class="lang-cs prettyprint-override"><code>public class MyClass
  {
      [IndexField(BuiltinFields.UniqueId)]
      public string UniqueId { get; set; }
  
      [IndexField(BuiltinFields.ID)]
      public int Id { get; set; }
  }
  </code></pre>
  
  <p>How to add item id to Solr index ?
  What should I add to schema.xml ?
  Is next string would be enough ? </p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;field name="_id" type="string" indexed="true" stored="true" required="true" /&gt;
  </code></pre>
  
  <p>If yes why Sitecore doesn't include it during build schema.xml file for Solr ?</p>
  
  <p>It seems that it could be an issue with duplication records in search result that I have as well :) </p>