The Go standard library is probably being too opinionated here, but it's in line with the general worse-is-better philosophy behind Go: simplicity of implementa

The Go standard library is probably being too opinionated here, but it's in line with the general worse-is-better philosophy behind Go: simplicity of implementation is more important than correctness of interface. In this case, the interface can even be claimed to be correct (according to the spec), but it cannot cover all use-cases. If my memory serves me right, we did use Traefik at work in the past, and I remember having this issue with some legacy clients, which didn't expect headers to be transformed. Or perhaps the issue was with Envoy (which converts everything to lowercase by default, but does allow a great deal of customization).