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:

• https://github.com/improbable-eng/grpc-web/blob/0c7a81a25d11e97972b5b6e6e61312fd7ad6413a/go/grpcweb/grpc_web_response.go#L74-L77
• https://github.com/improbable-eng/grpc-web/blob/0c7a81a25d11e97972b5b6e6e61312fd7ad6413a/go/grpcweb/grpc_web_response.go#L100-L103
• https://github.com/improbable-eng/grpc-web/blob/0c7a81a25d11e97972b5b6e6e61312fd7ad6413a/go/grpcweb/grpc_web_response.go#L141-L144
• 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 https://github.com/improbable-eng/js-browser-headers/issues/21 also.

[johanbrandhorst]

Still investigating this