Get the User-Agent header from flag instead of from the request

flag.userAgent is set from window.navigator.userAgent, which is the internal user agent set when the JSDOM instance is created. This ensures we are using the correct user agent.

lib/jsdom/living/xhr-utils.js

@@ -349,7 +349,7 @@ function createClient(xhr) {
       preflightRequestHeaders["Access-Control-Request-Headers"] = nonSimpleHeaders.join(", ");
     }
 
-    preflightRequestHeaders["User-Agent"] = requestHeaders["User-Agent"];
+    preflightRequestHeaders["User-Agent"] = flag.userAgent;
 
     flag.preflight = true;