Empact/roo — pull request #77 by pbonnell (2013-10-03 05:16:24+00:00)
Title: Fixes remote CSV filename from getting set to 'nil'.

Rails 4, Ruby 2.0.0, carrierwave, Heroku --
When creating a CSV with a URL ( `Roo::CSV.new(remote_csv_url)`) , was throwing
```ruby
TypeError (no implicit conversion of nil into String)
```
`make_tmpdir` was nil-ing the local `filename` by using both a local assignment and calling the filename instance var.