Ensure quantity is coerced to integer in OrderContents#add_to_line_item

It *might* come from a controller parameter, which is a string. The API's LineItemsController#create action does this, for instance.
[api] Cache-Control headers are no longer set per-request
[api] remove test for adding line items with an update call

To add a line item, you should make a request to POST /api/orders/:number/line_items