If we take a look at documentation http://apidock.com/rails/ActionView/Helpers/FormOptionsHelper/select we will see, that the select method accepts as options as html options, so as the second hash is not specified, the mentioned option will be ignored. But following will work:
<%= f.select :quantity, [[1, 1], [2, 2], [3, 3], [4, 4], [5, 5], [6, 6], [7, 7], [8, 8], [9, 9], [10, 10]], { selected: line_item.quantity }, { onchange: "$('#item-id').submit();" } %>