title: GetModifiedMembers returns empty array
body: <p>The code below always return an empty array from <code>GetModifiedMembers(object)</code>
  but <code>dx.GetChangeSet().Updates.Contains( foo )</code> returns true.</p>
  
  <pre><code>DataContext dx = new DataContext( string.Empty );
  MockLinqDataObject foo = new MockLinqDataObject();
  dx.GetTable( foo.GetType() ).Attach( foo );
  
  foo.PK = Guid.NewGuid();
  
  // always returns empty array
  ModifiedMemberInfo[] arr_Result = dx.GetTable( foo.GetType() ).GetModifiedMembers( foo );
  bool isOk = ( arr_Result.Length == 1 );
  return isOk;
  </code></pre>
  
  <p>Does anyone know what there is wrong?</p>
  
  <p>Thanks in advance?</p>