feat(GoogleMerchant): Allow to load products via scope.

[tvdeyen]

Currently all products are loaded from the database during application boot, because we yield the block when we
register a feed.

This is far from ideal! Especially if you want to boot the app to prepare specs or build assets on heroku or in a docker container.

I think we can agree that accessing the database during app boot is generally a bad idea.

Current fix

One could prevent accessing the database by using this little hack

feed.generator = ->(output) {
  SolidusFeeds::Generators::GoogleMerchant.new(
    Spree::Product.available,
    host: Spree::Store.default.url
  ).call(output)
}

but this does not feel quite right.

Proposed fix

Instead let's allow to pass a product_scope instead of products. By using available as default scope we do what most stores want anyway.

So, it now gets as simple as

SolidusFeeds::Generators::GoogleMerchant.new(
  host: "www.example.com", # You should pass this instead of using
  # the Spree::Store.current, which as well accesses the database
)

or pass your custom scope

SolidusFeeds::Generators::GoogleMerchant.new(
  product_scope: :solidus_mugs,
  host: "www.example.com"
)

[kennyadsl]

👍