Allow Spree::StockLocation and Spree::ShippingMethod to also be found by admin name.

Fix for 'undefined method with_indifferent_access for nil:NilClass' when not passing an address.

Add specs to finds stock location by name or admin name

Add specs to finds shipping method by name or admin name

Add specs to find stock location by name or admin name

Add specs to find stock location by name or admin name

Add specs to find shipping method by name or admin name

Fixes #33