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
+1 to moving to the assets guide.

This seems too detailed for the a release note entry. As noted in #16576, the release note entries should be short, to the point and scannable. We should aim to make it easy for people to answer the question "does this change affect me" and point them to elsewhere for more information if it does. Usually this is all contained in the pull request/commit (hence the link), but in this case we could add a link to the new section in the assets guide, something like [this](https://github.com/rails/rails/blame/master/guides/source/4_1_release_notes.md#L663)