feat(http1): add options to preserve header casing (#2480)

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

src/proto/h1/io.rs

@@ -160,7 +160,6 @@ where
                     cached_headers: parse_ctx.cached_headers,
                     req_method: parse_ctx.req_method,
                     h1_parser_config: parse_ctx.h1_parser_config.clone(),
-                    #[cfg(feature = "ffi")]
                     preserve_header_case: parse_ctx.preserve_header_case,
                     h09_responses: parse_ctx.h09_responses,
                 },
@@ -644,7 +643,6 @@ mod tests {
                 cached_headers: &mut None,
                 req_method: &mut None,
                 h1_parser_config: Default::default(),
-                #[cfg(feature = "ffi")]
                 preserve_header_case: false,
                 h09_responses: false,
             };