gateway: Handle If-Modified-Since a part of fix for #848

[mocchira]

leofs@cat2neat:leofs.1.3.5$ curl -v -H 'If-Modified-Since: Fri, 06 Oct 2017 07:36:53 GMT' http://localhost:8080/leofs-adm/CHANGELOG.md -o CHANGELOG.md.dl
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying ::1...
* connect to ::1 port 8080 failed: Connection refused
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 8080 (#0)
> GET /leofs-adm/CHANGELOG.md HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.47.0
> Accept: */*
> If-Modified-Since: Fri, 06 Oct 2017 07:36:53 GMT
>
< HTTP/1.1 304 Not Modified
< Connection: keep-alive
< Date: Fri, 06 Oct 2017 07:52:46 GMT
< Content-Length: 0
< Server: LeoFS
<
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
* Connection #0 to host localhost left intact

[yosukehara]

@mocchira Thanks