Do not check stock quantity twice when saving line item

Every time an user pressed the add to cart button spree was checking
stock in OrderPopulator and before saving the line item, through
Stock::AvailabilityValidator. I believe we only need to check stock on
the LineItem model
Fix api component orders spec

I believe the reason this spec was passing before was actually a Rails
3.2.13 bug which got resolved in 3.2.14.

  order.line_items << persisted_line_item

Looks like the code above wouldn't associate that persisted line item
with the given order on a Rails 3.2.13 app