Make headers title case for backward compatibility

[leiserfg]

Summary

Http headers are supposed to be case-insensitive (RFC 2616), but there are some implementations that don't normalize them.
I noticed it while migrating to uv, calls to an internal registry failed. A man in the middle server helped me to find that pip uses Title-Case while uv pip uses lowercase.

Test Plan

I tested uv with the same server and now it works fine.

[charliermarsh]

Interesting. I did some reading and it looks like this is disabled by default both out of spec compliance and due to performance risks. I wish we could enable this selectively so it didn't affect all users, since it seems like a minority of servers need this. Maybe an env var?

[leiserfg]

Regarding spec compliance, both are equally valid with the only difference being that correctly implemented servers will work with both but http1 servers will use the old way (Title-Case).
I don't think that changing the case of the header would have any impact on performance (I could be wrong tho).
What I fear is that if there are other servers broken the same way, the error coming from the server does not reflect it, you will just get a 401 or worse.

[leiserfg]

An example of a webserver not doing headers normalization: AWS lambda. Therefore local pypi using lambda like https://github.com/khornberg/elasticpypi will only work with uv if they handroll the normalization.