title: Non generic type Fields can not be used with type argument
body: <p>When I try to add <a href="https://www.elastic.co/guide/en/elasticsearch/client/net-api/current/source-filtering-usage.html" rel="nofollow noreferrer">Source Filter</a> to my <code>SearchRequest</code> I get error</p>
  
  <blockquote>
    <p>The non generic type 'Fields' can not be used with type arguments</p>
  </blockquote>
  
  <p>on <code>Fields</code>. here is the code snippet:</p>
  
  <pre><code>var request = new SearchRequest&lt;ElasticSearchJsonObject.Rootobject&gt;(Nest.Indices.Index("myindex"), Types.All)
      {
           From = 0,
           Size = 10,
           Query = query,
           Source = new SourceFilter
            {
                 Includes = "*",
                 Excludes = Fields&lt;ElasticSearchJsonObject.Rootobject&gt;(p =&gt; p.footer)
            }
      };
  </code></pre>
  
  <p>Error appear in Excludes Fields even when I try <code>Fields("footer")</code> directly.
  This is how documentation suggests.</p>
  
  <p>I am using Elastic Search 6.0.2 and NEST 6.0.1. </p>