Merge #1557

1557: Explicitly define ProxyFix options r=mergify[bot] a=brian-maloney

## What type of PR?
bug-fix

## What does this PR do?
This PR explicitly defines the options for the ProxyFix module, which fixes a regression in admin behind a reverse proxy.

### Related issue(s)
- #1309

## Prerequistes
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.

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

This is a bugfix, so not doc changes, and it's an extremely minor change.


Co-authored-by: Brian Maloney <3286425+brian-maloney@users.noreply.github.com>

core/admin/mailu/utils.py

@@ -40,7 +40,7 @@ class PrefixMiddleware(object):
         return self.app(environ, start_response)
 
     def init_app(self, app):
-        self.app = fixers.ProxyFix(app.wsgi_app)
+        self.app = fixers.ProxyFix(app.wsgi_app, x_for=1, x_proto=1)
         app.wsgi_app = self
 
 proxy = PrefixMiddleware()