Add test for CsvPreviewController#show for XHR request & unknown format
Add test for CsvPreviewController#show for XHR request & unknown format

The respond_to block and the rescuing of ActionController::UnknownFormat
was added in this commit to avoid XHR requests raising an exception. I'm
not sure whether this type of request is still being used, but it seems
sensible to add a specific test to ensure the fix is working as intended
- no test was added in the original commit.

[1]:
https://github.com/alphagov/whitehall/commit/3bf68b32e30d410edc5cd3cbfbb7a2bb8e4fe692

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall