Specs for CssLoader.file_name, tests correct fingerprint cutting

**premailer-rails** latest version on rubygems has an issue. If asset file name contains any digits, but not a fingerprint then ```CssLoader``` changes file name. For example if I have asset with file path ```company-4567.css``` then after passing through ```CssLoader.file_name``` it changes asset file name to ```company.css``` which is not correct.

I see that you fixed this issue in master branch already. So my fix is not needed, but I also added some specs to prevent this issue happens in future versions.