What's the problem?

Legacy, standard noncompliance client that do header in case sensitive way?

I am doing a mitm proxy for a binaryonly app to inject a auth header for our internal firewall.
When I get the header, the case is not preserved and broke the legacy app.

Apache mod_proxy, nginx, squid all do that in case preserving way. But there is no way to do this in go without rewriting the whole Round Tripper.

I saw the previous proposal to control the case of header have been rejected.
That's why I am proposing an alternative way to do this -- allow adding my own entry to the hardcoded list.

We aren't going to add new API to the standard library to support legacy clients that are not standards compliant. You will have to make your own copy of the net/textproto package, or run a transformation on the input stream.