Fixed "calling .set on destroyed object" error in gh-image-uploader tests
Fixed "calling .set on destroyed object" error in gh-image-uploader tests

refs https://github.com/TryGhost/Admin/pull/2041

- the pretender upgrade highlighted an issue with the component's progress event handler where it could fire and attempt to set the progress property after the component has been destroyed

TryGhost/Ghost-Admin,TryGhost/Ghost-Admin