in src/Transport.ts [778:786]
export function lowerCaseHeaders (oldHeaders?: http.IncomingHttpHeaders): http.IncomingHttpHeaders | null {
if (oldHeaders == null) return null
const newHeaders: Record<string, string> = {}
for (const header in oldHeaders) {
// @ts-expect-error
newHeaders[header.toLowerCase()] = oldHeaders[header]
}
return newHeaders
}