Return first css path when route index is nil

[shinnc]

I was using CriticalPathCss.generate to generate dynamic routes that aren't defined in critical_path_css.yml, and hit with this error TypeError: no implicit conversion from nil to integer.

The reason is because routes.index(route) is nil, and you cannot call [nil] in an array, hence css_paths.first is never called. Let me know if .to_i works, since it might seems too magical for some people. On the other hand, please let me know if I'm using the .generate method incorrectly...

[michael-misshore]

Thanks for the contribution. I'll get this pushed out as soon as I can.