title: DocumentDB ReplaceDocument Fails
body: <p>In Azure DocumentDB using .NET SDK, I get the following error when calling ReplaceDocumentAsync:</p>
  
  <p>"Errors":["The input content is invalid because the required properties - 'id; ' - are missing","The request payload is invalid. Ensure to provide a valid request payload."]</p>
  
  <p>It's a blog post scenario, when a new comment is added, I get the document, add the comment and call ReplaceDocumentAsync. Here's how I do it:</p>
  
  <pre><code>string query = "SELECT * FROM Posts p WHERE p.id = 'some guid'";
  
  var post = Client.CreateDocumentQuery&lt;Post&gt;(Collection.DocumentsLink, query)
  .AsEnumerable().FirstOrDefault();
  
  post.Comments.Add(comment);
  
  Document doc = Client.CreateDocumentQuery(Collection.DocumentsLink)
              .Where(d =&gt; d.Id == id)
              .AsEnumerable()
              .FirstOrDefault();
  
  var document = await Client.ReplaceDocumentAsync(doc.SelfLink, item);
  </code></pre>
  
  <p>Post class:</p>
  
  <pre><code>public class Post
  {
      public Post()
      {
          Comments = new List&lt;Comment&gt;();
      }
  
      public Guid Id { get; set; }
      public List&lt;Comment&gt; Comments { get; set; }
      ...
  }
  </code></pre>
  
  <p>What am I doing wrong?</p>