Redmine

This is an extension of #3614, which only dealt with making exported columns in PDF documents match the output of a custom issue query. That issue was closed with the changes in r2841.

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.