Ensure Location header is strictly a 'str', not subclass. Closes #5541 (#5544)

Author: lovelydinosaur

rest_framework/mixins.py

@@ -27,7 +27,7 @@ def perform_create(self, serializer):
 
     def get_success_headers(self, data):
         try:
-            return {'Location': data[api_settings.URL_FIELD_NAME]}
+            return {'Location': str(data[api_settings.URL_FIELD_NAME])}
         except (TypeError, KeyError):
             return {}