Selects 'state' as default zone type when there has not yet been a selection made.  We are already showing the 'add state' button in this case, so this commit makes the UI uniform ('state' radio is selected and 'add state' button is shown)

Fixes #3432
Do not check stock quantity twice when saving line item

Every time an user pressed the add to cart button spree was checking
stock in OrderPopulator and before saving the line item, through
Stock::AvailabilityValidator. I believe we only need to check stock on
the LineItem model

Fixes #3434
Add validation of shipping_category_id to Product. Fixes #3188

Conflicts:
	api/CHANGELOG.md
	api/app/helpers/spree/api/api_helpers.rb
	api/spec/controllers/spree/api/products_controller_spec.rb
	backend/spec/features/admin/orders/shipments_spec.rb
	core/CHANGELOG.md
	core/spec/models/spree/product_spec.rb
Fix spec broken by conflict.