Disappearing pricing attributes / product options on line item edit

The problem is the same as described in #1320526: line item update and this module requires the same logical patch!

The problem happens when you use the functionality to edit an already created line item. As stated out in the other issue:

1. I type some text in the textarea and then push "add to cart"
2. Cart page is fine (text is there) so I click Title (links to /node/4?line_item_id=34) to edit it.
3. If I change the text in textarea and push update cart my text is gone and cart now links to /node/4?line_item_id=35
4. If I click Title again, enter something into (now empty) textarea and click update cart the text is in place again.

In short — I can't actually EDIT a line item attributes preserving the Line Item ID, it gets incremented each time and also "loses" content 50% of times I try to edit it.

I'm not sure if it's related to this module or to Product Options, but they're connected anyway.

I'm trying to implement product customization so I need a text field per product (line item) and

The fix is quite simple and can also be found in the parent issue. I'll attach a patch for this module based on it.