title: Magmi product attribute_set changed to default on update
body: <p>I'm having an issue with Magmi for the product import.</p>
  
  <p>On the product creation import, the correct attribute_set is set for the products and everything is ok but on the second import with no attribute information (another csv), products attribute_set is changed to Default.</p>
  
  <p>What am I doing wrong here?</p>
  
  <p>Thanks</p>