According to RFC 9110, I believe headers are meant to be case-insensitive.

Field names are case-insensitive ...

Running the test suite in this project with a header of Content-Type yields errors. Would be nice to case-insensitively process those headers.

I understand most clients will lower-case headers on the way out, but we have been using this in machine-to-machine APIs that don't always get lower-cased on the way out.