Fix InMemoryUserSessionStore - user.shopify_user_id does not exist

[zzooeeyy]

What this PR does

Using ShopifyApp gem's InMemoryUserSessionStore doesn't work out of the box

shopify_user_id does not belong in ShopifyAPI::Auth::AssociatedUser , using id instead.

Added test around InMemoryUserSessionStore as well.

Reviewer's guide to testing

Set configuration user session repository to use InMemoryUserSessionStore, see that it fails before this fix, and can store sessions after this fix.

# my-app/web/config/initializers/shopify_app.rb
ShopifyApp.configure do |config|
#...
  config.shop_session_repository = ShopifyApp::InMemoryShopSessionStore
  config.user_session_repository= ShopifyApp::InMemoryUserSessionStore
#...
end

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

[mllemango]

❤️