removes request.script_name being prepended to asset urls

- fixes an issue where asset paths from within an engine are prepended with where the engine was mounted
- issue documented at https://github.com/jejacks0n/sprockets_engine
- clarification pending at https://github.com/rails/rails/commit/e6451a5599e92fa07e7c39562bacfd1824d199f8

I initially created an issue on rails/sprockets-rails, but identified the cause by digging into *why* it's happening.