iOS中http标头的自动大写？

Question

如何通过＆＃34; location＆＃34; （NSURLRequest或URLSession的HTTP标头中的所有小写字母）？我编码像

let session = URLSession.shared
let request = NSMutableURLRequest(url: url!)
request.httpMethod = "GET"
request.addValue("XYZLocation", forHTTPHeaderField: "location")
request.addValue("XYZLoc", forHTTPHeaderField: "loc")
print("Header : \(String(describing: request.allHTTPHeaderFields))")

始终打印Location: XYZLocation, loc: XYZLoc。我希望location:XYZLocation, loc: XYZLoc。为什么＆＃39; l＆＃39;总是在地点获得资本化？