Address comments on Gzip implementation

- don't mutate PATH_INFO in env, test
- test fallback content type matches Rack::File
- change assertion style
- make HTTP_ACCEPT_ENCODING comparison case insensitive
- return gzip path from method instead of true/false so we don't have to assume later
- don't allocate un-needed hash.

Original comments:

https://github.com/rails/rails/commit/cfaaacd9763642e91761de54c90669a88d772e5a

cc @jeremy
Leads to the question "how do I get pregenerated .gz files on disk?"  => Sprockets generates them by default.