The event name seems to have been incorrectly called `todo_due_date_changed`
instead of `todo_due_on_changed`. The API docs for webhooks don't mention
the correct event name, but the TODO json payload[1] seems to contain the
`due_on` field, aside from the fixture actually referring to
`todo_due_on_changed` event type.
[1]: https://github.com/basecamp/bc3-api/blob/master/sections/todos.md
