How to enable keep-alive in haproxy?

This is my haproxy.conf

global
    log             127.0.0.1 local0
defaults
    retries 3
    option redispatch
    timeout client  30s
    timeout connect 30s
    timeout server  30s
    option http-keep-alive
    http-reuse always

frontend web1
    bind *:8080
    option http-keep-alive
    mode http
    default_backend app1
backend app1
    balance     roundrobin
    option http-keep-alive
    mode http
    server a2 192.168.56.150:8000

curl result of origin server:

$ curl -vv http://ift.tt/2zRygES --keepalive-time 700
* About to connect() to 192.168.56.150 port 8000 (#0)
*   Trying 192.168.56.150...
* Connected to 192.168.56.150 (192.168.56.150) port 8000 (#0)
> GET /test HTTP/1.1
> User-Agent: curl/7.29.0
> Host: 192.168.56.150:8000
> Accept: */*
> 
< HTTP/1.1 200 OK 
< Etag: 720-6-59eeda80
< Content-Type: application/octet-stream
< Content-Length: 6
< Last-Modified: Tue, 24 Oct 2017 06:15:28 GMT
< Server: WEBrick/1.3.1 (Ruby/2.0.0/2015-12-16)
< Date: Fri, 27 Oct 2017 02:38:14 GMT
< Connection: Keep-Alive
< 
tests
* Connection #0 to host 192.168.56.150 left intact

curl result of haproxy server

$ curl -vv http://ift.tt/2xroSGn --keepalive-time 700
* About to connect() to 192.168.56.150 port 8080 (#0)
*   Trying 192.168.56.150...
* Connected to 192.168.56.150 (192.168.56.150) port 8080 (#0)
> GET /test HTTP/1.1
> User-Agent: curl/7.29.0
> Host: 192.168.56.150:8080
> Accept: */*
> 
< HTTP/1.1 200 OK 
< Etag: 720-6-59eeda80
< Content-Type: application/octet-stream
< Content-Length: 6
< Last-Modified: Tue, 24 Oct 2017 06:15:28 GMT
< Server: WEBrick/1.3.1 (Ruby/2.0.0/2015-12-16)
< Date: Fri, 27 Oct 2017 02:38:05 GMT
< 
tests
* Connection #0 to host 192.168.56.150 left intact

As you can see, there's no Connection: Keep-Alive in haproxy response, how can I make haproxy keep-alive??