Fix 'duplicate headers' bug in download original image

This contains the one critical bug-fix from #2971 (closed).

To test:
 - pick a single image to import (not MIF) with the image name containing a comma (rename with a comma if you can't find one). E.g. image,1.tiff
 - Import the image then try to download the Original file.

 - Select multiple images and try to 'Download' their original files. In the download dialog, add a comma into the zip name field before downloading. Check that downloads OK (with the comma replaced in the resulting zip name).