Fill address names combined when needed

In solidusio/solidus#3524 we introduced a new preference that allows
to use the combined version of firstname/lastname.
This commit updates checkout feature specs to reflect this preference.
Once the split version will be deprecated and removed, we can
probably also revert this commit and use the combined version only.

spec/features/checkout_spec.rb

@@ -403,8 +403,13 @@ def add_to_cart(item_name)
 
   def fill_in_address(address)
     fieldname = 'order_bill_address_attributes'
-    fill_in "#{fieldname}_firstname", with: address.first_name
-    fill_in "#{fieldname}_lastname", with: address.last_name
+    if Spree::Config.has_preference?(:use_combined_first_and_last_name_in_address) &&
+       Spree::Config.use_combined_first_and_last_name_in_address
+      fill_in "#{fieldname}_name", with: address.name
+    else
+      fill_in "#{fieldname}_firstname", with: address.first_name
+      fill_in "#{fieldname}_lastname", with: address.last_name
+    end
 
     fill_in "#{fieldname}_address1", with: address.address1
     fill_in "#{fieldname}_city", with: address.city