Include user-specified field selection issues CSV export
I blatantly plagiarized the code in r2841 to make the necessary code changes to the issues_helper to see if I could get CSV exports to similarly export the user-specified fields in a custom issue query. It works well for me, but I'm not familiar enough with either Ruby or Redmine itself to definitely say it's a good patch. I was pretty callous with my implementation.
I hope that others find this useful.