Fix for apps that will change env['PATH_INFO']

problem: So I am using font_assets along with sidekiq and it seems that sidekiq's web interface is changing the value of `env['PATH_INFO']` to a blank string, which then breaks things. 

caveat: I am not at expert at RACK so I am not sure that this fixes the problem at the root. Also, I am not really sure if sidekiq should be changing the `env`. 

This is a first attempt at a fix. The test suite passes. I am not sure if you want to add a test case for apps that change the value of 'env', so I didn't add one. 

Another possible fix might be to allow for env['PATH_INFO'] to hold a blank string but then again, I am not exactly sure what exactly the culprit is.