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
I added this logic to `gzip_file_path` so it doesn't stat when mime is wrong. Javascript doesn't return a `text/*` mime type, so had to special case it.