refactor websocket_wrapper.go with copyHeader func (#362)
refactor websocket_wrapper.go with copyHeader func (#362)

This commit makes websocket_wrapper.go file consistent with the changes
in grpc_web_response.go file, PR #359.
improbable-eng/grpc-web,improbable-eng/grpc-web,improbable-eng/grpc-web,improbable-eng/grpc-web,improbable-eng/grpc-web