rack 3.0 support

[pcai]

https://github.com/rack/rack/blob/main/UPGRADE-GUIDE.md includes many breaking changes. httpi explicitly depends on rack, and also uses it as part of its own public interface. Unfortunately, this interface has now changed in rack.

In rack 3, headers must be all lowercase. in rack 2, they don't have to be. so if we want to support rack 3, it appears we require a major breaking change to a public method HTTPI::Request#headers

This is only one example. There are very likely other breaking changes that affect httpi; I have not exhaustively reviewed their impact yet.

[pcai]

per #238 we would prefer to dedicate our efforts towards a migration to faraday rather than work on new features, closing