Blank out product SKU when cloning a product

This was effectively being done before for the product's sku (stored on the master variant) via the #duplicate_variant method, but now it needs to be done explicitly on the product in #duplicate_product

(cherry picked from commit 0f253bb2a0)

lib/spree/core/product_duplicator.rb

@@ -21,6 +21,7 @@ module Spree
         product.dup.tap do |new_product|
           new_product.name = "COPY OF #{product.name}"
           new_product.taxons = product.taxons
+          new_product.sku = ""
           new_product.created_at = nil
           new_product.deleted_at = nil
           new_product.updated_at = nil

spec/lib/spree/core/product_duplicator_spec.rb

@@ -69,6 +69,7 @@ describe Spree::Core::ProductDuplicator do
     duplicator = Spree::Core::ProductDuplicator.new(product)
     expect(new_product).to receive(:name=).with("COPY OF foo")
     expect(new_product).to receive(:taxons=).with([])
+    expect(new_product).to receive(:sku=).with("")
     expect(new_product).to receive(:product_properties=).with([new_property])
     expect(new_product).to receive(:created_at=).with(nil)
     expect(new_product).to receive(:unit_value=).with(true)

spec/models/spree/product_spec.rb

@@ -16,6 +16,7 @@ module Spree
         it 'duplicates product' do
           clone = product.duplicate
           expect(clone.name).to eq 'COPY OF ' + product.name
+          expect(clone.sku).to eq ""
           expect(clone.image).to eq product.image
         end
       end