2709: Validate proxy ip with PROXY_AUTH_WHITELIST r=mergify[bot] a=Diman0
## What type of PR?
bug fix
## What does this PR do?
The Proxy code validated the real client ip against the proxy auth whitelist. It should be the proxy ip that is checked. That is changed with this PR.
### Related issue(s)
- closes#2708
- #2692
## 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.
- [n/a] 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/workflow.html#changelog) entry file.
Co-authored-by: Dimitri Huisman <diman@huisman.xyz>
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