Canada post pws spree fixes

Two small changes I needed to make to get this working with my [canada_post_pws branch of spree_active_shippng](https://github.com/xorrbit/spree_active_shipping/tree/canada_post_pws).

We are using the carrier name as part of a cache key for caching shipping rates, so this adds a getter.

Also both origin_node() and destination_node() take location hashes as parameters, not location objects.