title: YamlDotNet - Serialization of Guid properties generates empty brackets
body: <p>I am using YamlDotNet library to serialize some objects in Yaml. I've met some problems with the serialization of Guid properties. 
  Serialization of Guid properties generates empty brackets ( ex: {} )</p>
  
  <p>See code below</p>
  
  <pre><code>Dim l As New List(Of Person)
  l.Add(New Person() With {.Firstname = "MyFirstName", .Lastname = "MyLastName", .Id = Guid.NewGuid()})
  
  Using sw As New StreamWriter("output.yaml", False)
      Dim serializer = New Serializer()
      serializer.Serialize(sw, l)
  End Using
  </code></pre>
  
  <p>this code will output : </p>
  
  <pre><code>- Id: {}
    Firstname: MyFirstName
    Lastname: MyLastName
  </code></pre>
  
  <p>With the class:</p>
  
  <pre><code>Public Class Person
      Public Property Id As Guid
      Public Property Frstname As String
      Public Property Lastname As String
  End Class
  </code></pre>
  
  <p>Am I missing something or is this an issue of the library ?</p>