Add test for backdrop not being called on update
Do not call backdrop on data-set updates

[fixes #67930714]
https://www.pivotaltracker.com/story/show/67930714

Backdrop create data-set should only be called when creating a new
data-set. It should not be called when udpating an existing data-set.
Merge pull request #87 from alphagov/prevent-backdrop-on-update

Do not call backdrop on data-set updates