Use correct protocol (#1425)

Currently we can run webpacker in either http or https mode.
But if we run webpacker in http mode but access it through a reverse proxy handling https, then it will break because the reverse proxy will have set HTTP_X_FORWARDED_PROTO to https and webpack-dev-server will try to understand the request as a secured one.

We must override X_FORWARDED_PROTO in the same way as we override the other headers.

lib/webpacker/dev_server_proxy.rb

@@ -11,6 +11,7 @@ def rewrite_response(response)
   def perform_request(env)
     if env["PATH_INFO"].start_with?("/#{public_output_uri_path}") && Webpacker.dev_server.running?
       env["HTTP_HOST"] = env["HTTP_X_FORWARDED_HOST"] = env["HTTP_X_FORWARDED_SERVER"] = Webpacker.dev_server.host_with_port
+      env["HTTP_X_FORWARDED_PROTO"] = Webpacker.dev_server.protocol
       env["SCRIPT_NAME"] = ""
 
       super(env)