title: $project or $group does not support <document>
body: <p>I'm trying to run aggregate with projection but i get <code>NotSupportedException: $project or $group does not support &lt;document&gt;</code>. I am running version 2.4.4 of driver with mongodb v3.4.</p>
  
  <pre><code>var filter = Builders&lt;T&gt;.Filter.Regex(x=&gt;x.Value,"/test/gi");
  
  var aggregate = collection.Aggregate()
         .Match(filter)
         .Project(x =&gt; new 
         {
            Idx = x.Value.IndexOf("test"),
            Result = x
         })
         .SortBy(x =&gt; x.Idx);
  </code></pre>
  
  <p>I thought <code>IndexOfCP</code> is <a href="https://github.com/mongodb/mongo-csharp-driver/blob/e7a7e39da5f46b4eca28bea8986c658e72eccf7b/tests/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectTranslatorTests.cs#L587" rel="noreferrer">supported</a>.</p>
  
  <p>What am i doing wrong here?</p>