when trying this out locally i recognized, that uploading another image-file to the same image content does not invalidate the cropped scales (the old cropped image is still shown)

this is only the case when accessing the image via `context/@@images/image/thumb`
`context/image_thumb` shows the new image scaled and uncropped

see https://github.com/plone/plone.scale/pull/3#issuecomment-28597087 for some explanation

we already tested both cases in `test_accessing_images` so i'd do the same in this test.

since i can't update your branch / this pull request i created a branch with your changes in this repository and modified the test there: https://github.com/collective/plone.app.imagecropping/commit/a25b15d92e156e8bdc998a31575a6482aa123aeb

@tomgross  could you merge this test into your branch and make it pass?