Update application.py

Author: tongyuze

msal/application.py

@@ -2,6 +2,7 @@
 import json
 import time
 import logging
+import platform
 import sys
 import warnings
 from threading import Lock
@@ -789,7 +790,7 @@ def _build_client(self, client_credential, authority, skip_regional_client=False
         client_assertion_type = None
         default_headers = {
             "x-client-sku": SKU, "x-client-ver": __version__,
-            "x-client-os": sys.platform,
+            "x-client-os": platform.system(),
             "x-ms-lib-capability": "retry-after, h429",
         }
         if self.app_name: