ProxyExchange discards all incoming request headers when making a request with a body.

Affects: spring-cloud-gateway-webflux:3.1.1

Setup
Define an MVC RestController that accepts a ProxyExchange as an argument to a request handler method, and use the ProxyExchange object to issue a POST request (or any request that contains a body) to an upstream service, using a body created (or manipulated) in the request handler method.

Expected Behaviour
The ProxyExchange forwards the original request headers, minus filtered sensitive headers, and Content related headers in the upstream request.

Observed Behaviour
The ProxyExchange discards all original incoming headers, appending only the X-Forwarded-For and X-Forwarded-Proto headers. This behaviour is in contrast to the behaviour observed when using a ProxyExchange object to make requests to upstream services that do not contain bodies (GET, OPTIONS, etc).

Start with the MVC RestController entry point that accepts ProxyExchange in spring-cloud-gateway-webflux:3.1.1, and reproduce a POST request whose body is created or manipulated by the handler. Trace how ProxyExchange builds the upstream request and compare it with the GET or OPTIONS path. Done means non-sensitive original headers and content-related headers are forwarded while X-Forwarded-For and X-Forwarded-Proto remain present.