Regression: When using BASIC auth the pre-authenticated header is not forwarded to gateway pod

[RockyMM]

The changes introduced in #1787 are seemingly incompatible with the built-in ingress controller, which is the Ingress-Nginx Controller maintained by Kubernetes community. While somewhat compatible with the "official" NGINX Ingress Controller maintained by the NGINX the company, there are some incompatibilities. The change from #1787 is one of them. Juding by the https://github.com/kubernetes/ingress-nginx repo, the proxy-set-header is always part of the location and is not used anywhere else, therefore it seems that it must be part of the location-snippet.

The clean way to configure proxy-set-headers in the Ingress-Nginx Controller is to create a ConfigMap with the headers and to reference them in the central nginx-configuration ConfigMap.

I decided actually to simply return the strings where they were, but also to keep them in the location where #1787 introduce them, in order to keep Ditto working with the Ingress controller that @mladBlum is using.