Check for files to be closed using ObjectSpace

This will end all the problem we have with too many open files error.

The real change is in https://github.com/thoughtbot/paperclip/blob/44da76c021b8f1f25fc552ee049a51d2a7326c58/test/helper.rb. The rest of the commit is to remove all the `file.close` that I added to mitigate the error ...