Make line_items attr handle more flexible on api

When importing orders or creating orders and line items at once the
line_items key on order payload would provide an array not a hash. The
API need to consider then that both hash and array might be provided and
handle it gracefully

Conflicts:
	api/spec/controllers/spree/api/orders_controller_spec.rb
Add sku to line item attr_accessible

Allow users to find variant by sku when importing orders

Conflicts:
	api/app/models/spree/line_item_decorator.rb