Add :event_id as default event property
Only set :timestamp event property if it isn't already set.
Make tests more maintainable by parsing JSON and only testing the values we are interested in.
Change event_id to event_uuid, and add tests for the behaviour when overwriting default property keys.
Prefix all default properties with `event_`, and change `event_uuid` to now allow overrides anymore.
Merge pull request #6 from Shopify/default_properties

Default properties