Move :selected_shipping_rate to association
Move :selected_shipping_rate to association

This takes the existing `selected_shipping_rate` method and turns it into a `has_one` association. This enables eager loading, for instance `Spree::Shipment.includes(:selected_shipping_rate)`, which can speed up certain queries and avoid n-query issues in some cases.
imella/spree,brchristian/spree,vinsol/spree,brchristian/spree,vinayvinsol/spree,siddharth28/spree,ayb/spree,ayb/spree,siddharth28/spree,vinsol/spree,vinayvinsol/spree,DarkoP/spree,vinayvinsol/spree,calvinl/spree,imella/spree,brchristian/spree,calvinl/spree,ayb/spree,vinsol/spree,DarkoP/spree,siddharth28/spree,DarkoP/spree,calvinl/spree,siddharth28/spree,vinsol/spree,DarkoP/spree,brchristian/spree,ayb/spree,vinayvinsol/spree,calvinl/spree,imella/spree