dragtable.js sets incorrect weights

Repeatable: always

Steps to repeat:
1. create a table with three items, initial weights: -1,0,1
2. reorder the first 2, using drag and drop. (leave the third)
3. submit the new weights

Expected results:
three different weights (don't care about the actual values)

Actual results:
-1,-3,-1

Obviously these weights are now ambiguous. There's no question about the first item, but with the new weights there is no longer a fixed order on items 2 and 3.