Add first test case for ProductSet
user
```ruby
require 'spec_helper'

describe Spree::ProductSet do
  describe '#save' do
    context 'when passing :collection_attributes' do
      let(:product_set) do
        described_class.new(collection_attributes: collection_hash)
      end

      context 'when the product does not exist yet' do
        let(:collection_hash) do
          {
            0 => {
              product_id: 11,
              name: 'a product',
              price: 2.0,
              supplier_id: create(:enterprise).id,
              primary_taxon_id: create(:taxon).id,
              unit_description: 'description',
              variant_unit: 'items',
              variant_unit_name: 'bunches'
            }
          }
        end

        it 'creates it with the specified attributes' do
          product_set.save

          expect(Spree::Product.last.attributes)
            .to include('name' => 'a product')
        end
      end

      context 'when the product does exist' do
        let!(:product) do
          create(
            :simple_product,
            variant_unit: 'items',
            variant_unit_scale: nil,
            variant_unit_name: 'bunches'
          )
        end

        let(:collection_hash) do
          {
            0 => {
              id: product.id,
              variant_unit: 'weight',
              variant_unit_scale: 1
            }
          }
        end

        it 'updates all the specified product attributes' do
          product_set.save

          expect(product.reload.attributes).to include(
            'variant_unit' => 'weight',
            'variant_unit_scale' => 1
          )
        end
      end
    end
  end
end
```
assistant