Spring Support multi-value X-Forwarded-Prefix headers

When spring-cloud-gateway adds an X-Forward-Prefix header to a request on which an X-Forwarded-Prefix header is already present, the second value is added to the header separating both using comma.

This is currently not supported by the ForwardedHeaderFilter. The Filter prefixes the path with the full value of the Header, adding the commas to the Path. I haven't tested this with Webflux, but looking at ForwardedHeaderTransformer it doesn't seem to support this either.

I first opened this as an issue on the spring-cloud-gateway tracker (https://github.com/spring-cloud/spring-cloud-gateway/issues/1772) after some discussion with @spencergibb we decided to move this over here.