gistfile1.txt

$ curl -G --head httpbin.org/response-headers \
>     --data-urlencode 'ETag=W/"foo-bar"'
HTTP/1.1 200 OK
Server: nginx
Date: Sat, 30 Jan 2016 14:08:45 GMT
Content-Type: application/json
Content-Length: 96
Connection: keep-alive
ETag: W/"foo-bar"
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

$ curl --proxy http://localhost:8123 -G --head httpbin.org/response-headers \
>     --data-urlencode 'ETag=W/"foo-bar"'
HTTP/1.1 200 OK
Content-Length: 96
Date: Sat, 30 Jan 2016 14:09:05 GMT
Via: 1.1 polipo
Server: nginx
Content-Type: application/json
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Connection: keep-alive