Change the base.rb active_shipping calculator to only do rate calculations for the stock_location
that is represented by the passed in object. We are dealing with Spree::Shipment and Spree::Stock::Package
objects only.

In researching this issue, it seems we are passing a Spree::Shipment object whenever promotion_adjustments
is called when adding items to the cart and on checkout, we are passing the Spree::Stock::Package object.

The crux of the change is at line 160 where I am limiting calculations to the object.stock_location_id
Clean up this condition a bit. We cannot deal with anything other than Spree::Shipment
and Spree::Stock::Package objects.
Merge pull request #84 from rterbush/splitship-calculator

Change the base.rb active_shipping calculator to only do rate calculations for the stock_location