Headers are lowercased

Hi,
I try to provide header with `-H "isDebug: true"` but header name is lowercased with `headerName := strings.ToLower(strings.TrimSpace(pieces[0]))` during [MetadataFromHeaders](https://github.com/fullstorydev/grpcurl/blame/46c38b351a226b11ec734b7ac2454065aa496693/grpcurl.go#L156C4-L156C63)

Any reason for this behavior?

Start in grpcurl.go at MetadataFromHeaders and the referenced strings.ToLower call. Trace how the -H value is parsed and inspect nearby project guidance for expected header casing. Done means the behavior's rationale is established and the issue has a clear decision about whether any change is needed.

Written by the indexing model from the issue text.