Unable to pass Authorization header from URLRequest in iOS

I'm having problems accessing the Authorization header when sending a GET request from URLRequest in Swift.

I've made a simple script that converts the $_SERVER parameter to JSON and outputs to the browser to test that I'm sending the Authorization header correctly.

Here's my PHP code:

echo json_encode($_SERVER);

exit;

Here's my .htaccess file

RewriteEngine On

RewriteBase /

SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1

RewriteCond %{HTTP:Authorization} ^(.+)$
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

When I send a request via PHP or using the Postman application, I get the following response:

 {
  "TZ": "Etc/UTC",
  "REDIRECT_REDIRECT_UNIQUE_ID": "WQOWeWALzYlxj8TEssOrHgAAACA",
  "REDIRECT_REDIRECT_SCRIPT_URL": "/",
  "REDIRECT_REDIRECT_SCRIPT_URI": "/",
  "REDIRECT_REDIRECT_HTTP_AUTHORIZATION": "12345678",
  "REDIRECT_REDIRECT_HTTPS": "on",
  "REDIRECT_REDIRECT_SSL_TLS_SNI": "/",
  "REDIRECT_REDIRECT_STATUS": "200",
  "REDIRECT_UNIQUE_ID": "WQOWeWALzYlxj8TEssOrHgAAACA",
  "REDIRECT_SCRIPT_URL": "/",
  "REDIRECT_SCRIPT_URI": "/",
  "REDIRECT_HTTP_AUTHORIZATION": "12345678",

   ....etc....

The REDIRECT_REDIRECT_HTTP_AUTHORIZATION and REDIRECT_HTTP_AUTHORIZATION parameters return the value passed in the Authorisation header as expected.

When I try the same request using URLRequest in Swift the REDIRECT_REDIRECT_HTTP_AUTHORIZATION parameter is empty.

Here's my Swift code:

var url = URL(string: urlString)!

var request : URLRequest!
request.httpMethod = "GET"

request.setValue(auth, forHTTPHeaderField: "Authorization")
request.setValue("test", forHTTPHeaderField: "Hello")

URLSession.shared.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in

do {
    guard let responseData = data else {return}

    guard let jsonData = try JSONSerialization.jsonObject(with: responseData, options: []) as? NSDictionary else {return}

    print(jsonData)

I get the following response:

{

    . . .

    "HTTP_HELLO" = test;
    "REDIRECT_HANDLER" = "application/x-httpd-ea-php56";
    "REDIRECT_HTTPS" = on;
    "REDIRECT_REDIRECT_HTTPS" = on;
    "REDIRECT_REDIRECT_HTTP_AUTHORIZATION" = "";
    "REDIRECT_REDIRECT_SCRIPT_URI" = "/";
    "REDIRECT_REDIRECT_SCRIPT_URL" = "/";
    "REDIRECT_REDIRECT_STATUS" = 200;
    "REDIRECT_URL" = "/api/index.php";
    "REMOTE_ADDR" = "149.12.15.210";
    "REMOTE_PORT" = 64235;
    "REQUEST_METHOD" = GET;
    "REQUEST_SCHEME" = https;
    "REQUEST_TIME" = 1493407944;

    . . .

  }

The REDIRECT_REDIRECT_HTTP_AUTHORIZATION parameter is empty.

I can see that the HTTP_HELLO header is set to "test" so the problem doesn't seem to be in the Swift code.

This issue only seems to affect requests sent using URLRequest in Swift.

I assume the problem is on the server side and possibly in the .htaccess file but I can't find a way to get this working correctly.

The server runs CentOS 7 WHM/cpanel. I'm able to make changes to the WHM/cpanel settings if required.

Any help would be appreciated as I can't see what could cause the REDIRECT_REDIRECT_HTTP_AUTHORIZATION to be empty.

Thanks.