ignore http2 header frame scheme field

[someview]

We have find some point about grpc-dotnet server behaviors:

1. when grpc client send grpc request with tls, its header frame like this:

···
scheme: https
···

2. a grpc proxy unload tls, it may not change appliction layer content.
3. however, when grpc server receive this header frame from grpc proxy, it treat this as https, so 503 error was occoured.

In fact, the contents of the appliction layer should not be affected by secure layer.so the header frame remain unchanged. So, should grpc-dotnet omit the https header frame scheme field verify about http or https value？ I think it's worthy. Most grpc implements in other languages do not vertify this .