[master] httpd: X-Influxdb-Version header should be set on *all* responses

[mark-rushakoff]

To aid with troubleshooting, the X-Influxdb-Version header should be set on all responses, even for unhandled routes.

$ curl -G -vvv http://localhost:8086/ping # Has header set
*   Trying ::1...
* Connected to localhost (::1) port 8086 (#0)
> GET /ping HTTP/1.1
> Host: localhost:8086
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 204 No Content
< Request-Id: 50543231-2749-11e6-8002-000000000000
< X-Influxdb-Version: unknown
< Date: Tue, 31 May 2016 16:04:11 GMT
<
* Connection #0 to host localhost left intact

$ curl -G -vvv http://localhost:8086/asdf # Missing header
*   Trying ::1...
* Connected to localhost (::1) port 8086 (#0)
> GET /asdf HTTP/1.1
> Host: localhost:8086
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 404 Not Found
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Tue, 31 May 2016 16:03:55 GMT
< Content-Length: 19
<
404 page not found
* Connection #0 to host localhost left intact