title: UpdateItem operation. Do not insert item on update
body: <p>I'am writing basic CRUD operations for working with DynamoDb using .NET SDK.
  In process of developing update operation I faced with problem that DynamoDb is insert a new item into my table if current doesn't exist.</p>
  
  <p>Is there any smooth (or any) way to specify in UpdateItem action to not add a new item to the table if it does not already exist? 
  (Without throwing any exception)
  Or should I write IsExist() method and call it before update?</p>
  
  <pre><code>var table = Table.LoadTable(Client, "users");
  var doc = new Document();
  doc["id"] = id;
  doc["name"] = "John";
  
  table.UpdateItem(doc, id);
  </code></pre>
  
  <p>Thanks.</p>