UnmarshalBinaryResponse is not using canonical form for http header keys

[averzicco]

The net/http package expect the headers to be in canonical form [1]. This is not defined in any standard and http headers are still case insensitive, but right now the header keys in the http.Response generated from UnmarshalBinaryResponse function are lowercase so response.Headers.Get("Header") doesn't return the expected value (it's still possible to retrieve it directly from the map with response.Headers["header"]).

I'll open a PR to fix this, I hope you accept contributions to this project.

[1] https://pkg.go.dev/net/http#Header