Remove some redundant code and make file names consistent

- Remove the `redirect_with_whitespace` redirect factory block. This was a hangover of a previous implementation of #16 that I only noticed post-merge.
- Rename `furl_request_spec` to `short_url_request_spec` to match the model. The naming inconsistency initially confused me when completing #16. Fix a small typo in a test's description too.