Fix file serialize on windows

This fixes the file.serialize state on windows.
Currently, it fails on the first run (with "Failed to change group to Administrators" and succeeds on runs afterwards.

salt/states/file.py

@@ -4243,6 +4243,14 @@ def serialize(name,
         return _error(
             ret, 'Neither \'dataset\' nor \'dataset_pillar\' was defined')
 
+    if salt.utils.is_windows():
+        if group is not None:
+            log.warning(
+                'The group argument for {0} has been ignored as this '
+                'is a Windows system.'.format(name)
+            )
+        group = user
+
     if merge_if_exists:
         if os.path.isfile(name):
             if formatter == 'yaml':