Update Proxy-Support check to be case insensitive (#79620)

RFC4559 does not specify that the Proxy-Support header value used to
determine if the proxy server will honour client server authentication
integrity is case sensitive. Updating the check to be case insensitive
to prevent failures when value is supplied using differences in case.

Fix #61414

Co-authored-by: Christopher Wood <chris@thetoxiczone.com>

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/AuthenticationHelper.NtAuth.cs

@@ -61,7 +61,7 @@ private static bool ProxySupportsConnectionAuth(HttpResponseMessage response)
 
             foreach (string v in values)
             {
-                if (v == "Session-Based-Authentication")
+                if (v.Equals("Session-Based-Authentication", StringComparison.OrdinalIgnoreCase))
                 {
                     return true;
                 }