Shipping methods determination ignore shipping category of assembly

[romul]

Package#shipping_methods https://github.com/spree/spree/blob/3-0-stable/core/app/models/spree/stock/package.rb#L68 is based on parts shipping categories, but store owner can set separate shipping category for assembly itself.

I propose to override https://github.com/spree/spree/blob/3-0-stable/core/app/models/spree/stock/package.rb#L64..L66 with:

module Spree
  module Stock
    Package.class_eval do
      def shipping_categories
        contents.map { |item| item.line_item.variant.shipping_category }.compact.uniq
      end
    end
  end
end

What do you think about it?

[JDutil]

looks good to me