Replace explicit shipping_address and billing_address attributes

Due to deprecations, with a previous commit we replaced `alias_attribute`
calls for these two attributes with `alias_method`, but the latter doesn't
fully cover the previous behavior.

When `shipping_address` or `billing_address` are passed as attributes,
the error `ActiveModel::UnknownAttributeError` is now raised. For this
reason, we're starting to replace them with the right association name.

We may want to revisit the possibility to still use `billing_address` and
`shipping_address` as providing these aliases doesn't seem to be fully
viable anymore.

sample/db/samples/orders.rb

@@ -7,14 +7,16 @@
 store = Spree::Store.first!
 
 orders = []
+binding.irb
+
 orders << Spree::Order.create!(
   number: "R123456789",
   email: "[email protected]",
   item_total: 150.95,
   adjustment_total: 150.95,
   total: 301.90,
-  shipping_address: Spree::Address.first,
-  billing_address: Spree::Address.last
+  ship_address: Spree::Address.first,
+  bill_address: Spree::Address.last
 )
 
 orders << Spree::Order.create!(
@@ -23,8 +25,8 @@
   item_total: 15.95,
   adjustment_total: 15.95,
   total: 31.90,
-  shipping_address: Spree::Address.first,
-  billing_address: Spree::Address.last
+  ship_address: Spree::Address.first,
+  bill_address: Spree::Address.last
 )
 
 orders[0].line_items.create!(