Setting OwinRequest.Host doesn't work as expected

[marc-hall-oak]

Hi

I am running an OWIN server (.net framework) behind a proxy. The OWIN app has the following middleware:

public override Task Invoke(IOwinContext context)
{
    var forwardedProto = context.Request.Headers.Get("X-Forwarded-Proto");
    if (forwardedProto != null)
    {
        context.Request.Scheme = forwardedProto;
    }

    var forwardedHost = context.Request.Headers.Get("X-Forwarded-Host");
    if (forwardedHost != null)
    {
        //context.Request.Headers.Set("Host", forwardedHost);
        context.Request.Host = new HostString(forwardedHost);
    }

    return Next.Invoke(context);
}

x-forwarded-* headers are set and this code executes. context.Request.Scheme is updated as expected. Setting context.Request.Host adds the Host header but context.Request.Host returns the unaltered host value (which would be localhost:xxxx.) I tried setting the Host header directly (commented out here) and it has the same effect. I've looked at the implementation of OwinRequest and it looks like it should just work.