- add mocked response for test
- move error generation into BUYRequestOperation
- renamed shopify error domain constant value (ensure uniqueness)

- switch from channelID to appID
- adjust API endpoints for products and collections
- adjust mocked responses

This adds a attributes to checkout. These objects are returned as an array of dictionaries, but expected as dictionaries when posting the checkout, so we're required to serialize the objects correctly. API now also returns the `note` on a checkout, so I have added a test for that.

This adds a attributes to checkout. These objects are returned as an array of dictionaries, but expected as dictionaries when posting the checkout, so we're required to serialize the objects correctly. API now also returns the `note` on a checkout, so I have added a test for that.