title: PetaPoco fails inserting record with Guid newid primary key
body: <p>I'm experimenting with Peta POCO ORM and I quite like it, however there is one thing I can't make work. I'm using MS SQL and table with uniqueidentifier as primary key. However when I insert new record, POCO throws an InvalidCastException trying to cast DBNull to Guid. I created a trivial example. MS SQL table definition :</p>
  
  <pre><code>    CREATE TABLE MyTable
  (
      id uniqueidentifier NOT NULL DEFAULT newid(),
      name nvarchar(50) NULL,
      surname nvarchar(50) NULL,
      CONSTRAINT pk_MyTable PRIMARY KEY (id)
  )
  </code></pre>
  
  <p>and then in C# I have:</p>
  
  <pre><code>[PetaPoco.TableName("MyTable")]
  [PetaPoco.PrimaryKey("id")]
  public class MyTable
  {
      public Guid id { get; set; }
      public string name { get; set; }
      public string surname { get; set; }
  }
  </code></pre>
  
  <p>and finally I try to insert new records:</p>
  
  <pre><code>using (var conn = new SqlConnection(myConnString))
  {
  conn.Open()
  using (var db = new PetaPoco.Database(conn))
              {
                  for (int i = 0; i &lt; 3; i++)
                  {
                      var table = new MyTable() { name = "oh my", surname = i.ToString() };
                      db.Insert(table);
                  }
              }
  }
  </code></pre>
  
  <p>Thanks for any help</p>