List order customer returns only once

[spaghetticode]

Customer returns are associated to orders via return items. This means that, if an order has multiple line items and a single customer return for all of them, then the customer return will be listed twice. Listing customer returns multiple times adds noise without any significant benefit, and can be misleading when calling order.customer_returns.size and similars.

Given this is an ActiveRecord feature, it doesn't make much sense to add a spec in the codebase for covering it, considering also that the spec would require the creation of many different records, but for the curious ones it can be verified with:

order = create(:order_with_line_items, line_items_count: 2)

customer_return = build(:customer_return)

return_items = (0..1).each do |n|
  customer_return.return_items << create(:return_item, inventory_unit: order.inventory_units[n])
end

customer_return.save!

customer_return.order.customer_returns.size

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[kennyadsl]

Makes sense, the spec could be useful to avoid future regressions on the same thing but I'm fine as is as well. Thank you, Andrea!