Merge #2659

2659: Doh.  r=mergify[bot] a=nextgens

## What type of PR?

bug-fix

## What does this PR do?

Fix a bug introduced in #2646. Without this email delivery from RELAYNET is broken (admin throws a 400 that leads to a ``451 4.3.2 Internal server error``)

### Related issue(s)
- #2646

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [ ] In case of feature or enhancement: documentation updated accordingly
- [ ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

core/admin/mailu/internal/views/auth.py

@@ -33,7 +33,7 @@ def nginx_authentication():
     for key, value in headers.items():
         response.headers[key] = str(value)
     is_valid_user = False
-    username = response.headers['Auth-User']
+    username = response.headers.get('Auth-User', None)
     if response.headers.get("Auth-User-Exists") == "True":
         if utils.limiter.should_rate_limit_user(username, client_ip):
             # FIXME could be done before handle_authentication()