Note the practical affects of this change:
- Dependency count with --features full dropped from 65 to 55.
- Time to compile after a clean dropped from 48s to 35s (on a pretty underpowered VM).
Closes#2388
Decouple preserving header case from FFI:
The feature is now supported in both the server and the client
and can be combined with the title case feature, for headers
which don't have entries in the header case map.
Closes#2313
