image_url from Search API results no longer yields a downloadable image

[pdurbin]

The Search API has a documented feature wherein a field called image_url appears in search results but this no longer works as of some performance optimizations that where made in the 4.2 timeframe regarding thumbnails. I discovered this while poking around in the code for #3559 and @landreev encouraged me to open an issue.

An example is "image_url":"https://demo.dataverse.org/api/access/dsCardImage/2" from http://guides.dataverse.org/en/4.6/api/search.html . Instead of an image you always get 204 No Content.

No one has complained that this image_url feature doesn't work any more. The only place where I'm aware that it's used is the proof of concept Android app I I knocked together a while ago: https://github.com/IQSS/dataverse-android . Here's a screenshots of how they where used:

[pdurbin]

I'll open this back up when I feel like there's demand for it. Closing.