Re-enable the sending of pre-announcement trailers

[johanbrandhorst]

Sometime between 1.9.2 and 1.18.0 the gRPC Go client started including the pre-announcement trailer headers in the header values read by the client. We should aim to provide the same experience, so I think we should turn on sending these. Specifically, it would mean removing the following lines from the proxy:

• grpc-web/go/grpcweb/grpc_web_response.go

  Lines 74 to 77 in 0c7a81a

  	// Skip the pre-annoucement of Trailer headers. Don't add them to the response headers.
  	if strings.ToLower(k) == "trailer" {
  	continue
  	}

• grpc-web/go/grpcweb/grpc_web_response.go

  Lines 100 to 103 in 0c7a81a

  	// Skip the pre-annoucement of Trailer headers. Don't add them to the response headers.
  	if strings.ToLower(k) == "trailer" {
  	continue
  	}

• grpc-web/go/grpcweb/grpc_web_response.go

  Lines 141 to 144 in 0c7a81a

  	// Skip the pre-annoucement of Trailer headers. Don't add them to the response headers.
  	if strings.ToLower(k) == "trailer" {
  	continue
  	}

• https://github.com/improbable-eng/grpc-web/blob/master/go/grpcweb/websocket_wrapper.go#L58-L61
• https://github.com/improbable-eng/grpc-web/blob/master/go/grpcweb/websocket_wrapper.go#L78-L81

I briefly tested simply removing these lines and running some tests but it appears the client is blocking on something, and I haven't yet had the time to investigate where.

[johanbrandhorst]

See improbable-eng/js-browser-headers#21 also.

[johanbrandhorst]

Still investigating this