Respect already set content-length header for HEAD request.

[TYsewyn]

Currently when making a downstream request from Spring Cloud Gateway, the content-length HTTP header is being overwritten by org.springframework.http.server.reactive.HttpHeadResponseDecorator. Ideally this decorator should not kick in when the header was already set manually, or otherwise.
Also see spring-cloud/spring-cloud-gateway#1210.

[rstoyanchev]

Come to think of it, we still need to wait for data buffers to be attempted to be written in order to release them. The question is what should happen if there are data buffers and an existing content-length header?

@TYsewyn I believe in your case there is no data to be written since you're simply passing through a response from the downstream which has a content-length but no data otherwise. So the change could be to respect an existing content-length if no content was written but otherwise just use the calculated length. That would keep it pretty close to the existing behavior with an improvement to use a suggested content-length when there is no data.

[TYsewyn]

Yep, I believe that was the use case of the issued ticket at SC Gateway. I think your proposed solution would be perfect. If the downstream service does send data then it would make sense to recalculate the content length since that's the content the gateway received.