fix unit_value initialization on the product duplicator

lib/spree/core/product_duplicator.rb

@@ -24,7 +24,7 @@ module Spree
           new_product.created_at = nil
           new_product.deleted_at = nil
           new_product.updated_at = nil
-          new_product.unit_value = nil
+          new_product.unit_value = %w(weight volume).include?(product.variant_unit) ? 1.0 : nil
           new_product.product_properties = reset_properties
           new_product.image = duplicate_image(product.image) if product.image
           new_product.variants = duplicate_variants

spec/lib/spree/core/product_duplicator_spec.rb

@@ -8,7 +8,8 @@ describe Spree::Core::ProductDuplicator do
            name: "foo",
            product_properties: [property],
            variants: [variant],
-           image: image
+           image: image,
+           variant_unit: 'item'
   end
 
   let(:new_product) do