* caching_middleware: fix gzip compression not triggered
* packages: If a client sets `Accept-Encoding: gzip`, the responseCache will
include `Content-Encoding: gzip` in all future responses, even
if a subsequent request does not set `Accept-Encoding` or another client
requests the file without setting `Accept-Encoding`.
Fix that.
* caching_middleware: use `test` instead of `match`
* add tests
* make code easier to understand
* make the regex more clear
